
Class _AX-i^ 

Book- ^Aii 

GopigM - 



COPYRIGHT DEPOSIT. 




Practical Talks on Electricity 



BY WILLIAM BAXTER, JR. 



PART II. 



Care and Management of Dynamos 
and Motors. 



THE ENGINEER PUBLISHING CO. 

355 Dearborn St., Chicago. 

1905. 



CL 






Copyright, 1905, by 
The Engineer Publishing Co. 



Table of Contents. 

Chapter XXVII. Using the Galvanometer. The principle 
of the galvanometer, how it is used for measuring small 
currents and in connection with the Wheatstone bridge 
for measuring resistances 205 

Chapter XXVIII. Care of Electrical Machines. Some oi 
the common difficulties with such machinery and the de- 
tection of causes of the trouble 213 

Chapter XXIX. Management of Generators. Use of the 
rheostat, details of Construction of different forms. Rea- 
son for use of the series- coil - irk compound wound ma- 
chines 219 

Chapter XXX. Running Two Generators in Parallel. 
How to determine the characteristic curves of generators 
and to find from these whether the generators will run 
satisfactorily in parallel or not 226 

Chapter XXXI. Connecting Generators in Parallel. 
Proper connections for dynamos which are to be run in 
this way, the method of throwing the generators into 
circuit and the reasons for each step 232 

Chapter XXXII. Changing the Voltage of Generators. 
The effect of changing speed, armature connections, and 
field current on the electromotive force generated by a 
dynamo 239 

Chapter XXXIII. Grounds and Short Circuits. How to 
test machines and line wires for grounds by the use ot 
lamps and the voltmeter ; the effects of grounds on the 
system. How to test for short circuits and to locate the 
point in a field coil where the short circuit will be found. 246 

Chapter XXXIV. Repairing Short Circuits in Arma- 
tures. Probable causes of short circuits, how the loca- 
tion is indicated in the generator and motor and how to 
repair it 255 

Chapter XXXV. Finding Broken Wires in Armatures. 
Indications of a broken wire and how to locate the break. 
Directions for repairing when found 203 



Chapter XXXVJ. Connection of Shunt-wound Motors. 
Connections of the starting box to the motor and to the 
line wire, the action of starting box and switches, and 
the inside connections for a number of kinds 269 

Chapter XXXVII. Changing the Speed of Motors. Fac- 
tors which affect motor speed and the different methods 
of changing speed for series and shunt wound machines. 277 

Chapter XXXVII. Motor Starters and Controllers. De- 
tails of construction and connection for various types. . . 285 

Chapter XXXIX. No Voltage and Overload Motor Start- 
ers. The connections and method of operation of safety 
devices on starting boxes 293 

Chapter XL. Motor Controllers. Giving the connection of 

speed control boxes of several types 303 

Chapter XLI. Reversing Motor Controllers. Connections 
and method of operation for changing the speed of a 
motor and for reversing its direction of running 309 

Chapter XLII. Motor Controllers for Printing Presses. 
Details of special devices used for motors on this class 
of work 316 

Chapter XLIII. Motor Starters with Magnetic Switches. 
Details of connection and action of these starters and 
the reasons for their use 3 2 4 

Chapter XLIV. Testing Electric Motors. Methods of 
testing the efficiency of the machine, including the meas- 
urement of the resistances of various parts. Efficiency 
by the electrical method 330 

Chapter XLV. Testing Electric Motors. Continued. 
Methods of testing for efficiency by mechanical measure- 
ments of power 339 

Chapter XLVI. Testing Electric Generators. Methods 
of finding the losses in various parts of the machine and 
the efficiency of the machine as a whole 347 

Chapter XLVII. Storage Batteries. Arrangements and 
methods of connection for batteries and battery systems 
and switch-boards 355 



206 



USING A GALVANOMETER. 



For detecting the presence of a current in a conductor, the 
simplest kind of a galvanometer will answer ; such instruments, 
which are commonly called current detectors, are cheaply made 
and some of them can he bought for 50 or 75 cents. For making 
measurements of resistance, it is necessary to have a more ac- 
curately constructed instrument. The simplest way in which a 
galvanometer can be arranged to measure resistance is shown 
in Fig. 144, but this method is seldom used, except for very high 
resistances, as it is not accurate. 




FIG. I44. 



In Fig. 144, G represents a galvanometer, B an electric bat- 
tery which furnishes the current for making the measurement, 
and C, a spool of wire the resistance of which we wish to ascer- 
tain. If the battery B is disconnected so that there is no cur- 
rent flowing in the circuit, the needle of the galvanometer will 
not be deflected, but will point toward the north, which in the 
figure is in the vertical direction. As soon as the battery is 
connected, a current will flow and the galvanometer will be 
deflected. 



USING A GALVANOMETER. 207 

Suppose it is deflected to the position indicated in the dotted 
lines ; then if we remove spool C and substitute for it resistance 
coils of different sizes, until we get a sufficient number in the 
circuit to cause the galvanometer needle to be deflected to the 
same position as with the spool, we know that the resistance in 
the two cases is the same. If, now, we add up all the resist- 
ances we have substituted for C and find that they make, say, 
io ohms, we know that the resistance of C is 10 ohms. 

This method, although simple, is not accurate, for several 
reasons : First, unless the galvanometer needle is of consider- 
able length, we cannot determine accurately the exact point to 
which it swings, either with the spool C or the measuring resist- 
ance we substitute for it. In the second place, unless the current 
is Aery weak, the needle will swing through a wide angle and, 
the farther it swings, the smaller the additional distance it will 
be advanced by a given increase in current. Thus, if the current 
is so weak as to cause the needle to swing through an angle 
of 3 or 4 degrees, doubling the current strength will increase the 
angle of swing to about double the amount ; but if the current is 
so strong as to cause the needle to deflect 85 degrees, then if the 
current is doubled, the deflection may not be increased to more 
than 86 degrees. Thus, it will be seen that, if the current is 
sufficient to deflect the needle through a considerable angle, the 
difference in the deflection produced by a small variation in the 
strength of the current will be so small as to be exceedingly 
difficult to detect. The third and last of the most serious objec- 
tions to this method is that, in order to be able to obtain ac- 
curate results — supposing that we can determine correctly the 
deflection of the needle — it is necessary for the voltage of the 
battery to remain absolutely constant, and it is almost a prac- 
tical impossibility to make such a battery. 

For measuring resistance the method illustrated in Fig. 145 
is the one commonly used. It is accurate, and easily understood. 
In this method, the galvanometer and the resistance coils with 
which the measurements are made are connected with one an- 
other and with a battery that furnishes the current, and the 
whole outfit is called a Wheatstone bridge testing set, or simply 
a bridge, or a testing set. 

To illustrate the principle of the Wheatstone bridge, sup- 



208 



USING A GALVANOMETER. 



pose the lines in Fig. 145 represent water pipes, and let the point 
A be higher than C , so that the current of water will flow from 
A to C by the force of gravity. Let G be a water meter placed 
in a pipe running between points B and D. Now it is evident 
that if B and D are on the same level, no water will pass through 
G, and consequently the indicator hand will not move; but if D 
is higher than B, there will, be a current of water through G 
from D to B, and, if B is higher than D, the current will 




FIG. I45. 



flow in the opposite direction. Thus it will be seen that we can 
determine whether B and D are on the same level by noticing 
whether the meter G indicates a current or not. If the two pipes 
A B C and ADC have a uniform inclination, the distance from 
A to B will be the same as that from A to D; but if the upper 
pipe has a steep decline near A and then runs comparatively 
level, the distance A B will be shorter than A D. 

This principle, as above illustrated in connection with water 
pipes, is that of the Wheatstone bridge, but instead of making 



USING A GALVANOMETER. 209 

use of differences in level, we make use of differences in elec- 
trical pressure, or potential, as it is called. To force an electric 
current from A to C requires an amount of pressure that is pro- 
portional to the resistance and the strength of the current. To 
force the current from A to B requires less pressure ; hence, we 
may say that the fall of potential between A and C is greater 
than between A and B. Suppose the resistance between A and 
D is 10 ohms, and between D and C also 10 ohms ; then, if the 
voltage acting to force the current from A to C is 20 volts, the 
current will be 1 ampere, and as the resistance from A to D is 
10 ohms, the fall of potential between A and D will be 10 volts. 

Now let the resistance from A to B be 20 ohms and from B 
to C also 20 ohms ; then the total resistance from A to C through 
a and x will be 40 ohms, and as the pressure acting between 
these points is 20 volts, the current will be ^2 ampere. Now, 
y 2 X 20, — i. <?., current in a times the resistance of a — is equal 
to 10 volts. Hence the fall of potential from A to B is 10 volts, 
and this is the fall of potential from A to D, so the electrical 
pressure at B and D is the same, and as a consequence no cur- 
rent will flow through the wire and the galvanometer G. 

From the foregoing it will be seen that, if we connect be- 
tween the points B and C the conductor whose resistance we 
desire to measure, and then insert between A' and B resistance 
measuring coils until no current flows through the galvanometer 
G, then the resistance at x and the resistance of the measuring 
coils at a will be equal. With this method, very accurate results 
can be obtained, as the smallest possible current passing through 
a sensitive galvanometer at G will cause the needle to deflect 
through a noticeable angle. Hence, we can determine with ex- 
treme accuracy the condition when the resistance x and the 
measuring coils a just balance each other. 

In order that the instrument may have a wide range of 
measurement, the resistances of the branches b and c are made 
so as to be varied. To illustrate the effect of such variations, 
suppose that b is made 100 ohms, and c is 1 ohm. Then the total 
resistance from A to C through the lower sides of the figure 
will be 101 ohms ; and if we have an electromotive force of 101 
volts, the strength will be 1 ampere, so that the fall of potential 
between A and D will be 100 volts. Now suppose we place a 



210 USING A GALVANOMETER. 

conductor in the side, x — that is, connected between B and C — 
and suppose that to obtain a balance so that no current flows 
through the galvanometer, we have to insert at a 10 ohms. Then 
we shall know that the resistance of the conductor at x is o.i 
ohm; or by adding this o.i to the 10 ohms at a, we shall have 
a total resistance from A to C, through a and x, of io.i ohms, 
and as the voltage between these points is 101, the current 
strength will be 10 amperes, and this multiplied by the resistance 
of 10 ohms, at a, gives just ioo volts, which is the same fall of 
potential as we found between A and D. Thus it will be seen 
that if the resistance of b is made ioo times as great as that of c, 
the resistance a will be ioo times as great as the resistance x, 
which we desire to measure. If we reverse the order of things, 
and make c ioo times as great as b, then the resistance at x 
will be ioo times as great as the measuring resistance at a. 

Testing sets for general use are arranged so that the resist- 
ances of b and c can be made equal, or b can be made ioo times 
as great as c, or c ioo times as great as b. By means of these 
changes, the capacity of the instrument is made 10,000 times as 
great as that of the measuring coils, and the smallest resistance 
that can be measured is the one-hundredth part of the lowest of 
the measuring resistances. The lowest of the measuring resist- 
ances is generally o.i ohm; so that the smallest resistance that 
can be measured at x with the instrument, is o.ooi ohm. The 
sum of all the resistance coils of the instrument is generally 
n,ooo ohms; so that the largest resistance that can be measured 
is one million, one hundred thousand ohms. 

Special bridges are made that have a greater range, and 
some are graduated for much smaller resistances, but with the 
same total range. 

Fig. 146 shows a Wheatstone bridge testing outfit. The bat- 
tery is placed in the right-hand side of the box, the galvanometer 
is at the back and the measuring resistances are connected with 
the rows of discs in front of the galvanometer. The conductor 
to be measured is connected with the binding posts on the left- 
hand side of the box. 

In making a test, after the conductor is connected with the 
binding posts, the switch key at the front is depressed, closing 
the circuit, and instantly the galvanometer needle is seen to 



USING A GALVANOMETER. 



211 



swing violently to one side. We now set the plugs in holes so 
as to connect some of the measuring resistances in the circuit, 
and again depress the switch key. If the needle now swings to 
the opposite side, we know that we have inserted too much re- 
sistance and proceed to cut some of it out, by changing the 




fig. 146. 



position of the plugs. When we get very near to a balance, the 
needle will swing more slowly out of the central position, and 
when a perfect balance is obtained, it will not move at all when 
we depress the key. 

Having reached this point, we add up all the resistances 



212 USING A GALVANOMETER. 

inserted in the circuit and then, by noting whether the resist- 
ances b and c are equal or not, we know whether this sum is 
the true resistance of the object we are measuring, or whether 
it is to be divided or multiplied by ten or one hundred. It will be 
noticed that next to the galvanometer, there are three plug holes 
on each side, one marked A and the other B, These are the b 
and c resistances of Fig. 145, and the figures on the disks state 
whether, with the plugs in certain holes, the reading is to be 
taken even, or whether it is to be multiplied or divided. 



CARE OF ELECTRICAL MACHINES. 



213 



CHAPTER XXVIII. 
Care of Electrical Machines. 

AFTER an electrical generator or motor has been in use for 
several years it is liable, like other machines, sometimes 
to act badly. It will be examined, and sometimes a 
correct conclusion is reached, but very often not. 

If a machine is old, it is more than likely that the shaft will 
be found out of center, and if this fact is discovered at a time 
when things are not working as they should, it is taken for 
granted that this is the cause of the trouble. For the present 
it will be sufficient to investigate just what effect displacement of 




fig. 147. 



fig. 148. 



the shaft can have; then, if the trouble with a machine so 
afflicted is not in the category of shaft disorders, we shall know 
that we must seek further for the cause of the complaint. 

Fig. 147 illustrates an armature of a two-pole machine which 
is out of center in one direction, and Fig. 148 shows another 
two-pole armafure out of center in a direction at right angles 
to that shown in the first figure. The condition shown in Fig. 
147 could be produced by a heavy armature running in rather 
light bearings for several years, and the side displacement of 
Fig. 148 could be produced by the tension of an extra tight belt. 
The mechanical effect of both these conditions would be to in- 
crease the pressure on the bearings, as the part a of the armature 
would be drawn toward the poles of the field with greater force 
than the opposite side. The downwc.d pull due to the attrac- 
tion of the magnetism, would be greater in Fig. 147 than the 



214 CARE OF ELECTRICAL MACHINES. 

side pull in Fig. 148, supposing both armatures and fields to be 
the same in both cases, and the displacement of the shafts equal. 
This difference is due to the fact that in Fig. 147 the magnetism 
of both poles is concentrated at the lower corners on account 
of the shorter air gap. Hence, both poles pull much harder on 
the lower side. 

In Fig. 148, the pull of the N pole is greater than that of 
the other, simply because in the latter the magnetism is more 
dispersed, but the difference in the density on the two sides will 
not be very great. If the bearings of a machine, with the arma- 
ture displaced as indicated, have shown any signs of cutting, or 
if they run unusually warm, their condition will be improved 
by putting in new bearings that will bring the shaft central. 

If the armature is of the drum type, the displacement of the 
shaft will have no effect upon it, electrically. This is owing to 
the fact that all the armature coils are wound from one side of 
the core to the other, and, therefore, at all times every coil has 
one side under the influence of one pole and the other side 
under the influence of the opposite pole, and if one side is acted 
upon strongly by one pole, it will be acted upon feebly by the 
other. 

If the armature is of the ring type, then the displacement 
of the shaft will affect it electrically, for in a ring armature the 
coils on one side are acted upon by the pole on that side only 
and as the magnetic field from one pole will be stronger than 
that from the other (that is, considering the action upon equal 
halves of the armature), the voltage developed in the coils on 
one side of the armature will be greater than that developed on 
the other side. 

In Fig. 147, if the brushes b b could be placed on the vertical 
diameter, as shown, the electrical action would not be interfered 
with, for on each side of the vertical line the magnetic action 
would be the same. But the reaction of the magnetism devel- 
oped by the armature current twists the magnetism around, so 
that the brushes have to be rotated around some distance from 
the vertical line; therefore, even in the case of Fig. 147 the 
electrical balance will be disturbed, if the armature is of the ring 
type. 

The effect of this disturbance of the electrical balance will 



CARE OF ELECTRICAL MACHINES. 215 

be that the brushes will spark badly, because the voltage of the 
current generated on one side of the armature will be greater 
than that of the current on the other side. Hence, when these 
two currents meet at the brushes, the strong one will tend to 
drive the weak one backward. If, while the armature is out of 
center, we wish to adjust the brushes so as to get rid of the ex- 
cessive sparking, all we have to do is to set them to the right 
of the center line in Fig. 148, so that the wire on the left side 
will cover a greater portion of the circumference than that on 
the right; or, what is the same thing, so that there will be more 
commutator segments between the brushes on the left side than 
on the right. In this way the voltages of the two armature cur- 
rents can be equalized, and the sparking can be cured, or very 
nearly so. 

In a multipolar machine, the displacement of the armature 
will have the same effect mechanically as in the two-pole type; 
that is, it will increase the pressure on the bearings and prob- 
ably cause them to cut, or at least to run warmer than they 
should. 

The effect produced upon the electrical action will depend 
upon the . way in which the armature is wound, or, more prop- 
erly speaking, upon the way in which the armature coils are 
connected with each other and with the commutator segments. 
Multipolar armatures are connected in two different ways, one 
of which is called the wave or series winding, and the other 
the lap, or parallel winding. (See Chapters XI and XII, Part I.) 

In the first named type of winding, the ends of all the coils 
on the armature are connected with each other and with the 
commutator segments in such a manner that there are only 
two paths through the wire for the current; therefore, these 
two armature currents pass under all the poles and the voltage 
of each current is due to the combined effect of all the poles. 
From this very fact it can be clearly seen that it makes no differ- 
ence what the distance between the several poles and armature 
may be, for if some are nearer than the others, the only effect will 
be that these poles will not develop their share of the total 
voltage, but whatever their action may be, it will be the same 
on the coils in both circuits. 

When a multipolar armature is connected so as to form a 



2l6 



CARE OF ELECTRICAL MACHINES. 



parallel or lap winding, the connections between the coil ends, 
and between these ends and the commutator segments, are such 
that as many paths are provided for the current as there are 
poles, and each one of these paths is located under one pole 
so that the voltage developed in it is proportional to the action 
of this pole. -The diagram Fig. 149 illustrates a six-pole arma- 
ture with the ends of the field poles, and the arrows a a, b b, c c, 
indicate the six separate divisions of the coils in which the 
branch currents are developed. 




FIG. I49. 



Now it can be clearly seen that as the armature is nearer 
to the lowest poles than to any of the others, the action of these 
will be the strongest. Hence, the currents a a will be stronger 
than the others and will have a higher voltage. These two cur- 
rents will be taken off the commutator by the brushes at the 
lower corners. These, same brushes also take off the currents 
developed by the action of the side poles, and which are in- 
dicated by the side arrows b c. These last two currents will 
be weaker and of lower voltage than the a a currents; hence, 
the latter will try to crowd them back and thus sparks will be 
produced at these brushes. 



CARE OF ELECTRICAL MACHINES. 217 

The two upper currents are weaker than the side ones, 
and their voltage is also lower, so that the current returning 
to the commutator through the brushes at the upper corners 
will not divide equally, but the larger portion will be drawn 
into the coils on the side; and as the upper coils will have to 
fight to hold their own, so to speak, there will be a disturbance 
of the balance that is required for smooth running. The result 
will be heavy sparking at these brushes. 

If these four brushes were shifted downward, the lower 
ones being moved more than the upper ones, points could be 
found where the sparking would disappear. This readjustment 
of the brushes would be the same thing for a multipolar ma- 
chine as the shifting to one side explained in connection with 
the action of a two-pole ring-wound armature. Multipolar ma- 
chines, however, are seldom made so that the brushes can be 
moved individually, so that we cannot count on correcting the 
trouble temporarily in this way. In the great majority of cases, 
if the brushes of a multipolar machine spark on account of the 
armature being out of center, the only cure is to reset the bear- 
ings, if they are adjustable, and, if they are not, to put in new 
ones. 

In two-pole machines we have seen that, if the armature is 
of the. drum type, the action of the brushes will not be affected 
by the displacement of the shaft, and this will also be the case 
in a multipolar machine, if the armature is wave or series wound. 
From this it will be inferred that there is a similarity between 
the two-pole drum winding and the multipolar wave winding, 
and such is really the case. The multipolar lap winding is the 
counterpart of the two-pole ring winding, and, in fact, a ring- 
wound armature will work perfectly in a machine with any 
number of poles, provided we place upon the commutator as 
many brushes as there are poles. 

If we made a ring armature and provided a number of 
different fields into which it would fit properly, one being two- 
pole, one four-pole, one six-pole, one eight-pole, and the others 
of greater numbers of poles ; then, if each machine had as many 
brushes as poles, and these were set in the proper position, the 
armature would run as well in one as in another, without re- 
quiring any changes in the connections between the armature 



2l8 MANAGEMENT OF GENERATORS. 

coils and the commutator. In fact, all we should have to do 
would be to remove it from one machine and place it in another 
and it would be ready to run. 

For multipolar machines the regular ring winding is not 
often used, because the coils have to be wound in place, and 
are, therefore, not so mechanical in appearance, and are more 
expensive to make. The formed coils almost, universally used 
for multipolar armatures have both sides on the outer surface of 
the core, and on that account, when they are connected into a lap 
winding, they will not operate perfectly with a number of poles 
different from that for which they are connected, but they will 
run, after a fashion, with any number of poles. That is, if we 
have two generators with four and six poles respectively, both 
using armatures of the same diameter, and both lap wound, if 
one armature gives out, we can use the armature of the other 
machine as a makeshift. An armature with a wave winding can- 
not be used except with a field of the number of poles for which 
it is wound. 

As it may sometimes be advantageous to change an arma- 
ture from one machine to another while repairs are being made, 
provided the dimensions of the machines are the same, it is de- 
sirable to know how to determine whether the winding is wave or 
lap connected. This is explained in Chapter XIII, Part I. 



MANAGEMENT OF GENERATORS. 210, 



CHAPTER XXIX. 

Management of Generators. 

ELECTRICAL generators are made of two types so as to 
develop two different types of currents. One of these 
maintains a constant voltage and the other a con- 
stant amperage. With the first-named type, the amperes in- 
crease and decrease in accordance with the demands of the cir- 
cuit, and in the second type the volts change in the same way. 
It must not be supposed that these two types of current represent 
different kinds of electricity. The difference between the two 
types is precisely the same as the difference between two streams 
of water, one of which keeps the pressure constant, and in- 
creases or decreases in volume, while the other keeps the volume 
constant and varies the pressure. 

Electric generators that maintain the amperes constant are 
called constant current generators, and those that keep the volts 
constant are called constant potential generators. Machines of 
the first-named type are used for arc lighting, while those of the 
second type are used for incandescent lighting, for operating sta- 
tionary motors and electric elevators, and also for electric rail- 
ways. In the early days of the electrical industry, electric gen- 
erators were called dynamos and the name is still used by the 
majority of people to designate arc light machines. The number 
of arc light generators used outside of lighting stations is very 
small in comparison with the number of constant potential gen- 
erators, probably not more than I per cent of the latter. On 
this account, what we have to say in this chapter will refer to 
the constant potential machines. 

Constant potential machines are of three different types, so 
far as the current is concerned. The simplest type is the shunt 
machine, which does not maintain the voltage absolutely constant, 
but suffers a slight reduction as the strength of the current in- 
creases. A well proportioned shunt generator will not vary its 
voltage more than 2 or 3 per cent from full load down to the 
lightest load, provided the speed at which it runs does not change. 
If the capacity of the generator is, say, 500 lamps, and it develops 



220 MANAGEMENT OF GENERATORS. 

i io volts with the full number of lights in operation, it will de- 
velop not more than 112 to 113 volts when only one lamp is 
burning. 

This would be the result if the speed did not change; but 
as a matter of fact, the speed will change, because the best gov- 
erned engines will run slightly faster with a light load than with 
a heavy one. A well governed engine will increase its velocity 
between 3 and 5 per cent between no load and full load, and 
this change in speed will cause the voltage of the generator to 
vary more than stated above. Hence, in actual practice, a well 
proportioned shunt generator driven by a well designed engine 
will vary its voltage from 6 to 9 per cent between full and light 
load, so that if, with all the lights burning, the voltage is no, 
with only one lamp in service, it will be anywhere from 116 to 
119 volts. 

In practice, however, the number of lights does not vary 
within such wide limits, the fluctuation being, as a rule, prob- 
ably not more than one-half as much, so that the actual variation 
in voltage is seldom more than 2 or 3 volts. This, however, is 
the variation of the pressure at the terminals of the machine; 
but if the lamps are located at a considerable distance from it, 
the fluctuation to which they will be subjected will be greater, 
because the portion of the voltage absorbed by the resistance of 
the line wires will increase as the current increases, and de- 
crease with current decrease. 

The only way in which these changes in voltage can be com- 
pensated for with a shunt generator is by varying the amount of 
rheostat resistance introduced into the field coil circuit. Many 
devices have been made that change this resistance automatically 
and some of them are simple and work well. If the number of 
lights in use is continually varying, the only way in which the 
voltage can be kept constant is by the use of one of these auto- 
matic regulators ; but if the number of lights remains constant 
for a considerable length of time, and the increase or decrease 
takes place at more or less regular intervals, then the field rhe- 
ostat can be changed by hand and the results obtained will be 
very satisfactory. 

The principle upon which a field rheostat acts can be ex- 
plained by the aid of Fig. 150 and then the way in which it 



MANAGEMENT OF GENERATORS. 



221 



should be manipulated to vary the voltage in any desired degree 
will be readily understood. In this diagram, the circle A repre- 
sents the generator armature, and M represents the field coils, 
while R is the rheostat. The bar C together with the contacts 
above it, which are connected with the several loops of the rhe- 
ostat, and the lever D, constitute the rheostat switch, by means 
of which the resistance is cut into or out of the field circuit. 
The lines P and N represent the line wires through which the 




fig. 150. 



current passes to the lamps. The points a and b represent the 
binding posts of the generator. From post a, a connection runs 
to bar C, and with the lever D in the position shown, the cur- 
rent that traverses the field coils M will also have to pass 
through all the resistance of R. 

Under these conditions the field current will be weak, and 
as a result the field magnetism will also be weak. Now the 
voltage developed in the armature at a given speed of rotation, 
will depend upon the strength of the field magnetism, being low 
when the latter is weak, and high when the latter is strong. 



222 MANAGEMENT OF GENERATORS. 

From this it will be seen that when the field current traverses all 
the resistance of R, the voltage will be the lowest obtainable at 
the speed at which the generator is run, and that with all the 
resistance of R cut out of the field circuit, the voltage will be 
the highest. The movement of D toward the right cuts out the 
sections of R consecutively, all of the resistance being removed 
from the field circuit when D rests on the last contact on the 
right-hand side. Any voltage between the highest and lowest 
can be obtained by placing D at points between the extreme 
right and left positions. 

Suppose a generator of five hundred lights capacity is used 
in a building in which about one-fifth of the lamps are in service 
during the day, and say four to five hundred at night from 6 to 
10 o'clock, after which hour the number drops off to about 
twenty, at which it remains until the next morning. In such a 
building, there would be two periods during the whole day when 
the rheostat would have to be manipulated to keep the lamps 
burning at the normal brilliancy. These periods would be when 
the night load begins, and when it drops off. From midnight, 
to about 6 o'clock the next evening, the number of lights would 
range between about twenty and, say, sixty; so that, if the rhe- 
ostat is set at midnight, when the small number of lights are in 
use, to about ]/ 2 volt above the normal pressure, it will be right 
for the day load, as increasing the lamps to sixty will not reduce 
the voltage too much. 

At the hour when the night load is about to commence, the 
attendant should keep his eye on the voltmeter and as fast as 
the pressure drops, the rheostat switch should be turned, so as 
to cut out the resistance and thus raise the voltage again to the 
standard point. In the course of half an hour or so, all the 
lights will be turned on, and the rheostat will not have to be 
looked after again until the time comes when the lights begin to 
be turned off. During this period, the voltage will rise as the 
lamps go out, and to keep it down to the standard, lever D will 
have to be moved toward the left, so as to cut more resistance 
into the circuit 

Field rheostats as actually used are not of the form illus- 
trated in Fig. 150, but they are connected in the field circuit in 
the way shown, and their action is as explained. Most rheostats 



MANAGEMENT OF GENERATORS. 



223 



are made in the shape of a box, and vary from a foot square to 
two or three times this size. The switch part is located on the 
front of the box and is arranged in the manner shown in Fig. 
151. The stud C takes the place of rod C in Fig. 150, and the 
switch D is the,tCounterpart of D in the first diagram. The cir- 
cle of contacts E are connected with the sections of the resist- 
ance in the same way as the contacts in the straight row above 
D in Fig. 150. At the ends of the circle of contacts, stops a and 
b are placed so as to prevent the switch from being moved en- 
tirely off the contact, and thus opening the field circuit. 



c9°/ h 


X 


8 I U 






O / (Dp 





/ ^\ 





/ 

% 


V 


°°oooooO u £> 


d 


B 



FIG. 151. 



If the field circuit is opened from any cause, the machine 
will stop generating; therefore, it is necessary not only to pro- 
vide these stops, but also to make all parts of the switch so 
perfect, mechanically, as to render defective contacts at any 
point next to impossible. It may also be well to add that all 
the connections in the field circuit, whether in the rheostat or 
elsewhere, must be carefully watched and not be allowed to get 
loose. A slightly imperfect joint in the field circuit may not stop 
the generation of current entirely, but it will lower the voltage, 
and if it is in such a condition that the contact becomes good 



224 MANAGEMENT OF GENERATORS. 

and bad by turns, the voltage will rise and fall, thus causing the 
lights to vary in brightness. A loose wire in a binding post, if 
so supported that it can swing or vibrate, may cause the voltage 
to dance up and down in such a manner as to lead to the con- 
clusion that something serious has happened to the generator. 

If the current developed by a generator is supplied to lamps 
that are near by, say within ioo feet, a shunt machine will meet 
the requirements perfectly; but if some lamps are within this dis- 
tance while others are 500 or 600 feet away, a compound gener- 
ator will give better satisfaction. 

As we have seen, the shunt machine can be made to keep 
the voltage about constant at its terminals by the aid of the field 
rheostat, but the voltage at the lamps under these conditions 
will not be constant, owing to the fact that a part of it is ab- 
sorbed in overcoming the resistance of the line wires. Now 
suppose that at a distance of, say, 500 feet from the generator 
there are two hundred lamps to be operated and that of these 
there will be only twenty or thirty in use during a portion of 
the time, while during the remainder of 'the day all the lamps are 
in use. Under these conditions, the voltage lost in driving the 
current through the line when all the lamps are used will be 
greater than when the number is small; hence, if the pressure is 
right when all the lights are in service, it will be too great 
when only a few are burning. 

If a compound wound generator is used, the voltage can be 
kept the same when the number of lights burning is large or 
small, because such a machine can be adjusted so that as the 
current generated increases, the voltage increases, and if this 
increase is just equal to the greater loss in the line, then at the 
distant lamps the pressure will remain the same at all times. 
At the nearby lamps, however, the voltage will rise as the num- 
ber of lights in service increases. In order to obtain the best 
results in such cases, the machine is so proportioned that the 
lamps about midway between the farthest and the nearest will 
have the same voltage at all times, and then the nearby ones 
will have slightly too great a voltage when the load is heavy, 
while the distant ones will not have quite enough, but the varia- 
tion in pressure at any of the lamps will not be sufficient to 
make a noticeable difference in the brightness of the light. 



MANAGEMENT OF GENERATORS. 225 

When a machine is adjusted so that the voltage at the ter- 
minals increases as the current increases, it is said to be over- 
compounded, a compound-wound machine being one which main- 
tains the voltage constant at the terminals. To make a 
generator compound or overcompound, the field is provided 
with two sets of coils, one being made of many turns of fine 
wire and the other of a few turns of large wire. The first set 
is the shunt coils, and the second is the series or compound- 
ing coils. The current through the shunt coils is derived from 
the terminals in the same way as in a simple shunt machine, 
but the current for the series coils is the entire current of the 
armature ; that is, the armature current does not pass out di- 
rectly to the external circuit, but first passes through the series 
field coils. 

Upon the strength of current flowing through them de- 
pends the magnetizing action of these series coils. Therefore, 
when the armature is generating a large current the series coils 
act more energetically upon the field. Thus it will be seen that 
the office of the series or compounding coils is to assist the shunt 
coils and we can further see that, if a certain number of turns of 
wire in these coils will enable them to assist the shunt coils 
sufficiently to keep the voltage just constant, then a greater num- 
ber of turns will so increase the assistance they give as to 
cause the voltage to rise as the current increases. The difference, 
therefore, between a compound and an overcompound genera- 
tor is simply that the latter has more turns of wire in the series 
coils. 



226 GENERATORS RUN IN PARALLEL. 



CHAPTER XXX. 

How to Find Out Whether Two Generators Can Be Run in 
Parallel. 

IT IS OFTEN desirable to determine whether two generators 
can be connected in parallel ; that is, whether they can be 

connected so as to feed into the same circuit. One who 
does not understand the subject would be likely to take it 
for granted that, if they are of the same size, or nearly so, they 
will work all right, but that, if one is much larger than the 
other, they will not. This conclusion, however, is far from being 
correct; in fact, the size has little, if anything, to do with the 
matter. 

For two generators to run together in the same circuit, all 
that is necessary is that they both develop the same voltage. 
The action of two electric generators working on the same 
circuit is the same as that of two pumps delivering water into 
the same pressure tank. If one pump has a cylinder 2 inches in 
diameter, and the other one is 2 feet, they will work in perfect 
harmony and each one will do its share of the pumping, if both 
develop the same pressure. If, however, the small pump, when 
running at its normal speed, can develop a pressure of 100 
pounds, and the large one can only work up to 90 pounds, then 
the small one will run above its velocity until its pressure drops 
to the same point as that of the large one, and it will do a great 
deal more than its proportion of the work. 

Between two pumps and two generators the comparison is 
not perfect, because the two pumps will work at practically a 
constant output, while the two generators will have to vary the 
work they do, probably two or three to one. If the two pumps 
were arranged so that their combined work would range from, 
say, 200 gallons to 1,000 a minute, they would furnish an exact 
parallel to the two generators. Now, if the amount of water 
delivered by the pumps was to be varied without changing the 
speed, then the capacity could be varied by changing the stroke 
of the pistons. This could be accomplished by having the crank- 



GENERATORS RUN IN PARALLEL. 227 

pins arranged so that they could be moved in or out from the 
center. 

With such an arrangement, it is evident that it would be 
possible fcr the devices by means of which the cranks are moved 
to be so proportioned that the stroke of both pumps would be 
changed alike, and on the other hand they could be so propor- 
tioned that the strokes would not change alike. If as the 
amount of water pumped varied, the strokes of the two pumps 
were changed by corresponding amounts, then, if the pressures 
of the two were the same for one stroke, they would be the same 
for all strokes. On the other hand, if the strokes are not 
changed by corresponding amounts, then the pressure of the two 
pumps would be the same for a certain length of stroke, but 
for all other lengths it would not, and as a consequence, there 
would be only one rate of pumping at which the two machines 
would operate properly. Above or below this point one pump 
would do more than its share of the work. If the small one did 
more for large outputs, the large one would do more for the 
smaller outputs. 

All the foregoing is true with respect to two generators ; 
that is, if they are to work together on a variable load, they 
must be able to develop the same pressure for corresponding 
increases or decreases in the strength of the current. This fact 
we can illustrate more clearly by the aid of Figs. 152 and 153. 
Suppose we have two generators, which, for the sake of sim- 
plicity, we will assume to be of the same capacity. Suppose that 
one of them develops a voltage of 112 when the current is prac- 
tically zero. When the current delivered is 20 amperes, suppose 
the voltage is in and at 35 amperes let it be no, while at the 
maximum output of 80 amperes it is 105 volts. 

If such is the performance of the machine, we can repre- 
sent it by a curve such as A in Fig. 152. In this diagram, the 
vertical lines measure off the amperes, and the horizontal lines 
the volts. The vertical line to the left represents zero amperes, and 
the top horizontal line represents 112 volts. Now, in the fore- 
going we have assumed that when the current is practically zero, 
the voltage is 112. Hence the curve A must start from the point 
where the zero ampere line and the 112-volt line intersect, and 
this is the starting point in the diagram. 



228 



GENERATORS RUN IN PARALLEL. 



We have further assumed that when the current is 20 am- 
peres, the voltage is in, and by looking at the diagram we shall 
find that the vertical line 20 and the horizontal line in meet 
the curve at the same point. In the same way the vertical line 
35, and the horizontal no meet at another point of curve A. 
The curve A is called the characteristic curve of the generator, 
and from examining it we can see at once the relation between 
the change in strength of current and voltage. 

Let us suppose now that we had another generator of the 
same size as the. one which gave the curve A, and that curve B 
is the characteristic of this second machine. By comparing 



Ii2 

111 
110 
109 
108 
107 
106 
















































^A 




























B 
















































js^a 




















































































































104 
103 
102 
101 












































































































































































100 




1 





2 





3 





1 





5 





6 





I 


u 


8 






AMPERES 
FIG. 152. 



these two curves, we find that the rate at which the voltage 
changes in the two generators is not the same, for that of ma- 
chine A drops 1 volt by the time the current reaches 20 amperes, 
while machine B does not lose 1 volt until the current is a trifle 
more than 40 amperes. Thus the B machine keeps up a more 
even voltage than the other. The two curves cross each other 
at the point a, and from this we see that, if we could keep the 
current constant at about 105 amperes, so that each machine 
would have to develop 52.5 amperes, the two generators would 



GENERATORS i(UN IN PARALLEL. 



229 



work together in a satisfactory manner; but if the current in- 
creased beyond this amount, machine B would do more than A, 
while for a decrease in current A would do the more work. 

Suppose that the characteristic curves of the two generators 
were as represented in Fig. 153, then for all strengths of cur- 
rent, A would develop a voltage that would be about the same 
amount higher than that of B. From this fact we would at 
once infer that, if we increase the speed of B sufficiently to ena- 
ble it to give the same voltage at, say, 10 amperes, as machine A, 



112 




































in 
















e 






^A 














110 

109 




































" 






V 


r"~ 








































> 
















10c 
























"1 












107 




























N 






































^=^ 




106 




































































> 


105 






































































104 














































































































































102 
101 
WO 




































































































) 


1 







J 




3 


) 


1 


j 


5 


J 


G 


[) 


? 





8 



AMPERES 
FIG. 153. 



it would give the same voltage as the latter with any other 
strength of current. Hence, these two generators would work 
together with a varying current and at all times each one would 
do its proper share of the work. 

By the foregoing process we ean determine whether two 
generators of the same capacity will work well together, and 
by the same means we can determine whether machines of 
widely different capacities can work together. To accomplish 
the latter result, all we have to do is to draw the characteristic 
curves to different scales, so far as the amperes are concerned. 



230 GENERATORS RUN IN PARALLEL. 

To illustrate this point more clearly, suppose that machine B 
works up to only 40 amperes, then if we draw its characteristic 
with the same scale for the amperes as we use for machine A, 
the curve would be only one-half the length of curve A, and we 
could not tell from looking at the two whether they would agree 
or not. 

If, however, we were to use a scale twice as great for the 
amperes of the small machine, then its characteristic curve would 
be the same length as that of A. Thus it will be seen that the 
curve B can be the characteristic of a machine that gives a 
maximum current of 40 amperes, provided we take the divisions 
on the horizontal, ampere scale, to indicate 2Y2 amperes instead 
of 5. We can further make B the characteristic of a 20-ampere 
machine by making the divisions of the horizontal scale measure 
1J/4 amperes. Thus it will be seen that to compare the charac- 
teristic curves of generators of different capacities, all we have 
to do is to so proportion the scales for the amperes that both 
curves will be of the same length. 

It will be noticed that in these diagrams, the vertical scale, 
which measures the volts, runs from 100 up, instead of from 
zero. We do this so as to represent the volts on a larger scale 
and thus cause the curves to drop faster. If we used the same 
scale as for the amperes, 5 volts to a division, the curves would 
run so nearly parallel with the horizontal lines that we could not 
determine the volts as accurately as with the larger scale. 

It may be said that, although the foregoing explanation of 
the way in which we determine whether two generators will run 
together, is simple enough, it is of no value except to those who 
know how to obtain the characteristic curves of the machines. 
This is very true, but it is a simple matter to obtain the char- 
acteristics, as will be presently shown. Testing of every kind 
is simply the art of measuring, and if you have the proper in- 
struments, and know how, it requires but little more ability to 
make a test of an electric generator and obtain its characteristic, 
than to weigh a pound of cheese or measure a piece of steam 
pipe. The writer has seen many apprentice boys, 16 to 18 years 
old, who could obtain these curves as accurately as any one. 

The course of "procedure is as follows: Obtain an ammeter 
of sufficient capacity to measure the maximum current of the 



GENERATORS RUN IN PARALLEL. 23 1 

machine, and also a voltmeter of proper capacity. Arrange a 
sufficient number of lamps or resistance coils to carry the full 
current. The generator should be driven by an engine that runs 
with as little variation in speed as possible. 

Connect the voltmeter with the terminals, and the am- 
meter in the main line, so as to measure the total current gen- 
erated at any time. Obtain a speed counter so as to get the 
speed of the armature when the instruments are read. Rule 
a sheet of paper in the same manner as Figs. 152 and 153, so 
as to mark on it the readings as obtained. Having done all 
this, you first start the generator and obtain the voltage of the 
machine with the main line open, that is, with zero current. 
Make a dot on the zero ampere line, on the ruled paper, at a 
point which indicates the volts shown by the voltmeter. At 
the same time that the volts are read, take the speed of the 
armature, and make a note of it. 

Now cut in lamps until the current rises to 5 amperes, 
and again take the voltmeter reading, and the speed. Mark this 
reading on the 5-ampere line, and then cut in more lamps so 
as to increase the current to 10 amperes. With this current 
read the voltmeter and the speed again and mark the result on 
the 10-ampere line. Proceed in this way until the current has 
been increased to the maximum amount, and you will find that 
the result is a number of dots, as shown on both sides of curve 
B in Fig. 153. By connecting these points you will get a zigzag 
line. 

This line will not show the true relation between the 
volts and the amperes, for, if it did, it would indicate that the 
action of the machine is very irregular. A curve drawn through 
these points, so as to strike a general average, will be the 
actual relation between volts and amperes. The irregularity 
in the actual measurements is due to the difference in the speed 
of the armature at the instants when the readings are taken, 
and also to the fact that as the pointers of the instruments 
vibrate, to some extent, it is not possible to get the exact results. 
To make as accurate a test of a machine as possible requires 
three men, one to read the volts, one for the amperes and one 
for the speed. In this way, by working at a given signal, all 
the readings can be taken very nearly at the same instant. 



232 



GENERATORS RUN IN PARALLEL. 



CHAPTER XXXI. 
Connecting Generators in Parallel. 

WHEN two or more generators are connected with a 
switchboard so as to feed into the same circuit, it is 
necessary to start and stop them in a certain way in 
order to avoid trouble; and, while in operation, several points 
have to.be looked after to secure satisfactory results and also 
to prevent accidents. What has to be done depends to some 
extent upon the type of machines — that is, whether they are 
shunt or compound wound. 





fig. 154. ■■■ 

Fig. 154 .is a diagramatic illustration of two shunt-wound 
generators connected with the same distributing mairis. The 
lines -B D represent the bus bars located at the back of the 
switchboard, and these are in reality nothing but the ends of the 
distributing mains through which the external circuit is supplied. 
The two generators are connected with these bus bars by means 
of the switches S, and, as will be noticed, all these switches are 



GENERATORS RUN IN PARALLEL. 233 

open so that both machines are entirely disconnected from the 
circuit. This is the way they should be when the generators 
are not running. 

To start up, the machines are set in motion, without turn- 
ing the S switches, and as can be seen, under these conditions 
the only path for the current generated in the armatures A A 
is through the field coils, each machine having its own field coil 
as a circuit. When the generators are running at full speed, the 
rheostat of one of them is adjusted so that the voltmeter indi- 
cates the proper voltage. This having been done, the two switches 
5 S are closed so as to connect the generator with the buses B D, 
and thus with the external circuit. 

We now turn our attention to the second generator, and ad- 
just its rheostat so that the voltage is a trifle higher than that of 
the first machine, say i volt more. Having done this, we close 
the 5 ^ switches so as to connect the second machine with the 
circuit. The next point to observe is the amount of current 
delivered by each generator, which should be its proportion of 
the load. Thus, if both machines are of the same size, both 
should develop equal amounts of current. If one machine is 
twice as large as the other, then the currents should be in the 
ratio of two to one. To be able to read the currents of both gen- 
erators without trouble, there should be provided on the switch- 
board two ammeters, one for each machine. 

If one machine is found to be delivering more than its share 
of the current, which is likely to be the case, it will show that its 
voltage is slightly too high, and to adjust it to the proper point 
all we have to do is to turn the field rheostat R so as to cut in 
a little more resistance. It is probable that when the second 
generator is cut into the circuit, the adjustment of the voltages 
will be imperfect, for the reason that the voltage of the machine 
already in circuit is that which corresponds to the strength of 
current it is delivering, while the voltage of the second gen- 
erator is that corresponding to a zero current. Now the voltage 
will drop as the current increases, and will rise as the current 
decreases ; hence, when the second machine is cut in, the voltage 
of the first one will increase, for now there are two machines to 
develop the same current; therefore, the current in the first one 
will be reduced, thus causing its voltage to rise. 



234 GENERATORS RUN IN PARALLEL. 

While the voltage of the first generator will increase, that 
of the second one will drop, because the current generated by- 
it will increase. We stated above, that the second machine is 
adjusted so that its voltage is slightly higher than that of the 
first one, before it is cut into the circuit, and as can now be 
understood, we make it a little higher so as to compensate for 
the rise in the voltage of the first generator, as well as the drop 
in that of the second. In making this allowance, however, we 
have to use our judgment as to what it should be and it is not 
likely that we shall hit the nail exactly on the head every time; 
so, in the majority of cases, we have to make a second adjust- 
ment after the two machines are connected with the circuit. 

When the generators are stopped, both must be cut out of 
the circuit, for if they are left connected, trouble may result 
when the next start is made. In addition to this it is not safe to 
stop the machines without cutting them out of the circuit, even 
if both are run by the same engine. If we desire to stop one of 
the generators, the only proper course is to open the switches 
5 ^ before the engine is stopped. If this is not done, as soon 
as the speed of the generator that is being stopped reduces a 
.trifle below the normal, its voltage will be so far reduced that 
current from the other generator will run back through it, and 
thus drive it as a motor. 

This is also the reason why it is necessary to disconnect 
both machines when they are stopped ; for if they are left con- 
nected, then in starting up the next time one machine may pick 
up its voltage sooner than the other one and current from it 
will not only pass out to the line but would also drive the other 
machine as a motor. 

Another reason why it is necessary to cut the generators 
out of the circuit when they are stopped is that if they are con- 
nected, they may not be able to 'pick up the current," as it is 
called. This is true even when only one machine is used. The 
reason why the machine may not pick up the current is that the 
external circuit may be closed through a low resistance, so that 
the machines will have to start under a full load, and under 
such conditions, generators will not always build up electro- 
motive force. 

Suppose a generator is started with the main circuit closed, 



GENERATORS RUN IN PARALLEL. 235 

and that the resistance is so low that the maximum current 
could be driven through it with the full voltage, then the cur- 
rent flowing through the armature with a small voltage would 
be quite strong. Now there is always a small amount of mag- 
netism left in the fields of a generator, and this is sufficient to 
develop a small voltage, which will drive a comparatively strong 
current through the main circuit, but the current that it will force 
through the field coils will be next to nothing, as these have a 
high resistance. The result of this difference in the strength of 
the armature and the field coil current would be that the mag- 
netism developed by the field coils would be insignificant, while 
that developed by the armature would be so far in excess of it 
as to completely overpower it, and thus prevent the machine 
from building up. 

As a rule, when the current stops, all the devices in the 
circuit that are operated by it are cut out, so that if a generator 
were started with the 5" 5 switches closed, the chances are that 
it would pick up, but it might happen that all the devices were 
connected with the circuit, and then the result would probably 
be different. 

When the generators are compound wound, the connections 
with the circuit are made as in Fig. 155. As will be seen from 
this diagram, the only difference between shunt and compound 
wound generators is that the latter have an additional set of 
field coils. We say set, because in these diagrams, the coil M 
represents all the shunt coils on the field, which may be one or 
two or even more, in a two-pole machine, while in a multipolar 
generator they would be equal in number to the number of poles. 
The coil M represents what are called the series, or compound- 
ing coils, and these will be the same in number, as a rule, as the 
shunt coils. 

Examining these diagrams of the generators in Fig. 155, we 
shall find that the M coil is connected in the same way as in Fig. 
154, but the m coil is so connected that all the current generated 
in the armature passes through it and thus the field is magnetized 
by the combined action of the two coils M and m. 

It will be noticed that from the point a, where the shunt coil 
connects, a wire runs down to the switch, and when this is closed, 
point a is connected with bus E. This is the connection for both 



236 



GENERATORS RUN IN PARALLEL. 



generators. From the end of the series coils m, a wire d runs 
to the switch, and through it connects with bus B, while the wire 
leading from the right-side armature terminal is connected 
through the switch with bus D. The bus E is called the equal- 
izing bus, and the wires which connect it with the point a are 
called the equalizing wires, or connections. The object of these 
connections, and the E bus, is to assist in keeping the currents of 




fig. 155. 



the generators equal. The way in which they accomplish the 
result can be made clear by the aid of Figs. 156 and 157. 

These diagrams are drawn to show the connections of the 
armature and the series coils m m only ; the shunt coils being left 
out so as to simplify the drawing. Fig. 156 shows the connec- 
tions just as they are in Fig. 155, while Fig. 157 shows them as 
they would appear if the equalizer bus and connections were not 
used. By examining Fig. 155 it will be seen that the effect of 



GENERATORS RUN IN PARALLEL. 



237 



the connection through the equalizing bus is to connect the 
points a of the two machines and this is what Fig. 156 accom- 
plishes in a simpler manner. 

Suppose that in Fig. 156 the armature on the left side gen- 
erates 40 amperes, while the one on the right generates 20, then 
the sum of the two currents will be 60 amperes, and this will 
pass through wire c. Now the current on leaving c will divide 
through the two m m coils in amounts that will be in proportion 
to the resistances of these coils. If both coils have the same 
resistance, each one will take half the current, that is 30 amperes. 
Thus we see that while one of the armatures generates 40 am- 





fig. 156. 



fig. IS7- 



peres and the other one 20, the currents passing through the two 
m m coils are of the same strength, namely, 30 amperes. 

If the connections were as in Fig. 157, it can be seen that 
the currents through the m m coils would not be equal but would 
be the same as those through the armatures ; that is, one would 
be 40 and the other 20 amperes. Now, under the latter condi- 
tions, the armature generating the stronger current would have 
its field magnetized to a greater extent by the action of its m 
coils, and thus its voltage would be increased so as to further 
increase the current; while the generator developing the weak 
current would have its field strengthened to a lesser degree by 
its m coils ; hence, the effect of these coils, if connected as in 
Fig. 157, would be to magnify the irregular ict ; on of the ma- 
chines, so that if one tended to do more than its share of the 
work, the increased effect of its m coils would cause it to do still 



238 GENERATORS RUN IN PARALLEL. 

more. By using the equalizing connection of Fig. 156, the m 
coils act to equalize the action of the generators, for no matter 
what the strength of the current through the armatures may be, 
it will be equally divided through the field coils m m. 

In starting compound generators, one is connected with the 
circuit first, just as with shunt machines. Then the second 
machine is started and adjusted so that its voltage is slightly 
lower than that of the first machine. This being done, the left- 
hand switch ^ is closed, so as to make the equalizing connection. 
The next switch to be closed is the center one, which places the 
m m coils of the two generators in parallel relation just as in 
Fig. 156. When this connection is made, the voltage of the first 
machine will drop slightly, and that of the second one will in- 
crease, for as soon as the connection is made, the current passing 
through the m coil of the first generator will be reduced to one- 
half its strength, thus slightly reducing the strength of the field, 
while through the m coil of the second machine the current will 
increase from zero to the same strength as that flowing through 
the m coil of the other generator, thus increasing the voltage of 
the second machine. 

We now close the right-hand switch and thus completely 
connect the second machine with circuit. If the two machines 
are not now taking their proper shares of the load, we adjust 
the rheostats so that they do, by increasing the resistance in 
the circuit of the shunt coils of the one that is doing more than 
its share, or reducing the resistance in the other. 

In stopping, if the switches are separate, they should be 
opened in the reverse order to that in which they are closed; 
that is, the one that is closed last should be the first one to be 
opened. As in the case of shunt generators, the machines should 
be started before they are connected with the circuit, and must 
not be adjusted until running at full speed; and in stopping they 
must be cut out of the circuit before the engine is stopped. 



CHANGING THE VOLTAGE OF GENERATORS. 239 



CHAPTER XXXII. 
Changing the Voltage of Generators. 

GENERATORS such as are used to furnish current for 
incandescent lamps are called constant potential genera- 
tors from the fact that they maintain the voltage, or 
potential constant regardless of how the current strength may 
vary. As a matter of fact, they do not keep the voltage abso- 
lutely constant, but the variation is so slight, in well-regulated 
machines, that for all practical purposes it can be regarded as 
constant. 

Constant potential generators are made either shunt or com- 
pound wound. A simple shunt generator has its field magnetized 
by coils that are connected in shunt relation to the armature. 
A compound generator has its field magnetized by two sets of 
coils, one being in shunt to the armature, and the other in series 
with it. The shunt coils are made of many turns of fine wire, 
and the series coils are made of a few turns of large wire. Fig. 
158 shows the way in which the shunt field coils and the arma- 
ture are connected in a simple shunt generator. In a compound 
machine the only difference is that the armature current instead 
of passing directly to the line wires L V , first passes through 
coils of wire wound upon the field magnets. 

Constant potential generators are made so as to develop a 
certain voltage at a certain speed, but it is a very difficult matter 
to so proportion a machine that it will give the required voltage 
at exactly the speed desired. For example, if we start out to 
make a generator that will develop a voltage of 115 at one thou- 
sand revolutions per minute, we may find upon testing it that 
the speed required to give this voltage is 983 revolutions per 
minute. The next machine made from the same patterns, and 
as nearly a duplicate of the first one as possible, may require a 
speed of 992 revolutions per minute to develop the 115 volts. 

It would not be desirable to mark each machine at the exact 
speed required, because there would be no uniformity; there- 
fore, a field regulator is provided by means of which the voltage 
can be adjusted within certain limits and, with this, the gener- 



240 



CHANGING THE VOLTAGE OF GENERATORS. 



ator can be run at one thousand revolutions per minute and the 
voltage will be 115. In Fig. 158 the field regulator is shown at 
B. This regulator is simply a resistance which is provided with 
a switch by means of which more or less of the resistance may 
be cut in or out of the field coil circuit. It varies the voltage of 
the machine by increasing or decreasing the strength of the field 
current. 

Generally field regulators are made of such capacity that, 




fig. 158. 




fig. 159. 



by inserting all the resistance in the circuit, the voltage can be 
reduced from 10 to 15 per cent, and in small generators, even as 
much as 25 per cent. This range of adjustment is provided be- 
cause it is not always practicable to run the machine at the 
proper speed. Thus, if it is rated at 1,000 revolutions per minute, 
it may not be possible to run it any nearer to this mark than 950 
without going to the expense of getting new pulleys. This re- 
duction in speed would lower the voltage from 115 to about 109; 
but if the field regulator is of large capacity, the generator will 
probably be able to develop the full voltage at even a lov/er 



CHANGING THE VOLTAGE OF GENERATORS. 24I 

velocity than 950, for the machine is likely to be so proportioned 
that it will give the rated voltage at the rated speed with about 
one-half the field regulator in service. If the machine has to be 
run above the rated speed, the voltage can be cut down by insert- 
ing more of the field regulator resistance. 

From the foregoing it will be seen that any generator can 
be adjusted to give a somewhat higher or lower voltage than 
that at which it is rated, by simply setting the field regulator. 
If we desire a higher voltage we cut out resistance; and for 
a lower voltage we add resistance. A further variation in voltage 
can be obtained by changing the speed of the generator, but a 
very great reduction in the voltage cannot be so made because, 
if the reduction is too great, the voltage will be so low that it 
cannot force through the field coils as much current as is neces- 
sary to cause the machine to act. 

By increasing the speed the voltage is increased and the 
more the speed is increased the higher the voltage; so that the 
increase in voltage by increasing the speed is only limited by 
the speed that is practicable and by the strength of current that 
the field coils can carry without burning out. If the voltage is 
increased, the current through the field coils will be increased, 
so that the increase in voltage will be greater than the increase in 
speed, from the fact that we shall have an armature running at 
a higher speed in a stronger field. Owing to this fact, the in- 
crease in voltage will not be limited by the speed at which the 
armature can be safely run, for long before this speed is reached 
the strength of the current flowing through the field coils will 
be all that they can safely stand. 

Without changing the speed of the armature, the voltage of 
the machine can be increased by connecting the field coils in 
parallel, as in Fig. 159. With this connection the strength of 
current passing through the field coils will be doubled, and if 
the wire will stand this increase without overheating, the voltage 
of the machine, at the same speed, will be increased 20 to 70 per 
cent, according to the density to which the field is magnetized 
with the regular connection of the field coils. If the coils cannot 
carry the increased current without overheating, an additional 
resistance can be connected in the circuit; that is, the resistance 
of the regulator B can be made greater. If this additional re- 



242 



CHANGING THE VOLTAGE OF GENERATORS. 



sistance is not at hand, the speed of the armature can be reduced 
until the field current becomes weak enough not to injure the 
coils. 

In the foregoing, several ways of changing the voltage of a 
generator are shown, but it will be noticed that in every case 
the variation is not very great, and it may be said that in gen- 
eral, it is not practicable to vary the voltage more than 70 per 
cent without reconstructing the machine; that is, if the normal 
voltage of the generator is 100 it cannot be increased to more 
than 170, and it cannot be reduced to less than 60 volts. 

As multipolar generators have four or more sets of brushes, 




fig. 160. 



it has been assumed by some inexperienced men that by prop- 
erly connecting these brushes, the voltage could be varied 
through a wide range. Such is not the case, however, and an 
attempt to make changes in these connections may lead to 
serious results. We will show why the desired result cannot be 
accomplished, and also what the actual result is liable to be. 

In Fig. 160 the armature and commutator of a four-pole 
machine are shown diagramatically. The outer circle represents 
the armature, and the inner circle C represents the commutator. 
The four brushes of such a machine are connected with each 
other as shown, the two side brushes with line wire a and the 
top and bottom brashes with wire b. The current entering 
through a follows the connecting wires cc to the side brushes, 



CHANGING THE VOLTAGE OF GENERATORS. 



243 



and after traversing the armature passes through the wires d d 
to line wire b. The path of the currents through the armature 
is indicated by the arrows e e and f f, and from these it will be 
noticed that each current flows through one quarter of the 
armature wire only. 

Now, it is natural to suppose that, if we were to connect 
the brushes in the way indicated in Fig. 161, the current enter- 
ing through brush a would follow the path of arrow c and come 
out through brush b', and that, if it were then conveyed to 
brush a , by means of a connection e, it would once more pass 
through the armature along the path indicated by arrow d, and 




fig. 162. 



fig. 163. 



:ome out at brush b. If the current would follow this path, its 
voltage would be doubled, for the voltage developed in the path 
d would be equal to that developed in path c. The difficulty in 
the way of realizing this result is that the connection e not only 
enables the current generated in c to pass to brush a', but also 
enables that generated in the quadrant spanned by e to return 
upon itself, as shown in Fig. 162. In this diagram it will be 
seen that the current generated in quadrant d' flows from a' 
to b' and thus the connection e simply serves as a short circuit 
for this portion of the armature; and a large machine with this 
connection would soon be disabled, as the heat developed in the 



244 CHANGING THE VOLTAGE OF GENERATORS. 

short-circuited portion of the wire would heat it to the burning 
point in a few minutes. 

Armatures can be connected so that the electromotive forces 
generated in the several sections are added to each other, and 
when so connected they are said to have a series, or wave wind- 
ing. But an armature wound so that the several e.m.f.'s are not 
added, cannot be made to give a higher voltage by changing the 
brush connections. The way in which armatures are connected 
for series or for parallel winding is illustrated in Fig. 163. If 
the armature is parallel-connected, or lap wound, as it is called, 
the current entering, through wire a will pass through the coils 
i> 3, 5> 7> 9 an d IX > m both the upper quadrants; and through 
the connecting wire c it will reach brush a' and then flow through 
the coils 2, 4, 6, 8, 10 and 12, and in this way the currents will 
reach the side brushes b V after traversing one-quarter of the 
number of coils on the armature. This is the case with a four- 
pole armature; with a six-pole the currents would pass through 
one-sixth of the number of coils, and so on for a greater num- 
ber of poles. 

If the armature is series-connected, or wave wound, the cur- 
rent from a will pass through coil 1, and then by a cross connec- 
tion (not shown in the diagram) will reach coil 2, and from the 
end of this coil by another cross connection will return to coil 3, 
from which it will pass to coil 4. Thus the current will cross 
from one side of the_.armature to the other until it reaches coil 
12, from which it will pass to brush b' . With this winding the 
connection c carries a part of the current to brush a , from 
which it enters coil 2 and follows the same path as the current 
entering at brush a. The only object of the connection c is to 
provide more brushes through which the current can enter and 
pass out, and thus prevent the undue heating of the brush ends. 
If the connection c is removed and also the brushes a and b, the 
action of the machine will not be interfered with in the least. 

In a series-wound armature the path of the. current. may be 
better illustrated by Fig. 164, but to properly understand this- it 
must be remembered that the current does not pass through all 
the coils in the quadrant c and then, through all those in quadrant 
<?', but through one coil in one quadrant and then through a coil 
in that opposite, and finally reaches the brush b. 



CHANGING THE VOLTAGE OF GENERATORS. 



245 



While a series-wound armature can be run with two brushes 
and deliver its full current, a parallel-wound armature, if used 
with two commutator brushes, will deliver only a portion of its 
full current. This can be understood from Fig. 165, in which, 





fig. 164. 



fig. 165. 



as there are only two brushes, a and b, the currents generated in 
the sections c and d have no outlet, and as the e. m. f.'s are in 
opposition to each other, they neutralize each other, so that only 
f he current generated in section c finds an outlet. 



246 GROUNDS AND FIELD SHORT CIRCUITS. 



CHAPTER XXXIII. 

Grounds and Field Short Circuits. 

WHEN the insulation between, an electric circuit or ma- 
chine and the ground becomes impaired, so that an 
electric connection is made with the ground, the circuit 
or machine is said to be grounded. If the electrical connection 
so established is perfect it is called a complete or dead ground, 
and if the connection is imperfect it is called a partial ground. 
Overhead line wires become grounded by rubbing against limbs 
of trees through which they pass or against the walls of build- 
ings into which branch connections are run, and in various other 
ways. 

Line wires are, as a rule, covered with an insulating en- 
velope, and to form a ground this covering has to be rubbed 
away by the chafing of the wire against the surface with which 
it comes in contact. In underground wires, ground connections 
are formed by the impairment of the insulating covering, either 
by the shifting of the wires, by the chemical action of gases, 
or by injuries inflicted by workmen when digging in the vicinity 
of the conduits. 

One ground in a circuit will cause no damage, because the 
current cannot escape through such a leak unless there is an- 
other connection through which it can get back into the circuit 
All the current that passes out of the generator through the 
positive wire must return to it through the negative; therefore, 
no current can leave the circuit proper, at one point, unless it 
car* find its way back a* some other point. 

Although a single ground cai: do no damage, it is inadvi- 
sable to permit it to exist, for it is alwaye possible for the sec- 
ond ground to form when least expected; and as soon as it does, 
there will be more or less serious trouble, according to the posi- 
tions of the two grounded points. Tests for ground connections 
can be made in a simple manner, and in every case, where the 
distributing lines run any distance and specially if so situated 
that there is a decided liability of their being injured, tests should 



GROUNDS AND FIELD SHORT CIRCUITS. 247 

be made every day. The apparatus required for making such 
tests is to be found in any place where a generator is installed, 
and it can be put in proper position in a few hours ; after it is 
once installed the daily tests can be made in a few minutes. 

For ground testing, the general arrangement of apparatus is 
illustrated in Fig. 166, in which L V represent the bus bars on 
the switchboard, or if there is no switchboard, as may be the 
case in a small plant, they may be taken to represent the main 
distributing wires, from which the branch circuits are taken. A 
represents the armature and M the field of a simple shunt- 
wound generator, R being the field regulator. B is the main 
switch for connecting the generator with the line wires. If the 
generator is compound wound it will make no difference in the 
connections of the ground detecting apparatus. From the wires 
leading from the generator to the main switch B, two wires, d d, 
are run to contacts e e' , of a small switch s, which latter is con- 
nected with one of the terminals of an incandescent lamp /. 
The other terminal of this lamp is connected with the ground as 
indicated at G. To make this ground connection, the wire can 
be attached to a water pipe, care being taken that a good metallic 
contact is obtained. 

To find whether there is a ground in any part of the entire 
circuit, the main switch B is closed so that the current of the 
generator may feed into the working circuit. From this it will 
be understood that the test is to be made while the machine is 
running and feeding the circuit. The small switch is now moved 
so as to connect with e, and then so as to connect with e . If 
when in either position the lamp / does not light up, we know 
that there is no ground in any part of the circuit; at least, no 
ground sufficiently bad to permit a current of any magnitude to 
pass through it. 

If the generator delivers a current at an e. m. f. of no volts, 
we can determine the existence of even imperfect grounds by 
substituting for the single lamp / several lamps of much lower 
voltage, their combined e. m. f. being no volts. Thus we could 
use two 55-volt lamps or four 25-volt lamps, these being con- 
nected in series, so that the current would pass through all of 
them, one after the other. Each one of these lamps should be 
provided with a small switch to short-circuit it. 



248 



GROUNDS AND FIELD SHORT CIRCUITS. 



With this arrangement of a number of lamps in series, we 
first close the switch s, with all the lamps in the circuit, placing 
it on e. If the lamps do not light up, we cut out one, and if the 
others still remain dark, we cut out another one, and so on until 
only one lamp is left in the circuit. If with this single lamp in 
service no light is produced, we then cut all the lamps back into 
the circuit, and move switch ^ to contact e' , and repeat the test. 




fig. 1 66. 



In this way an imperfect ground can be detected, because while 
the leakage current may not be sufficient to light up a single 
no-volt lamp, it may be capable of producing at least a visible 
light in a 55 or 25-volt lamp. 

Two wires, d d, are used to enable us to determine on which 
side of the circuit the ground is located. Suppose that there is a 
ground on the main L, or on one of the branches leading from 
it; then, since the contact e is connected with L, it follows that, 



GROUNDS AND FIELD SHORT CIRCUITS. 249 

if switch j is placed on e , only a small current will pass through 
the indicating lamp /; for it will be only that due to the slight 
difference in resistance between the ground connection and the 
portion of the line L between the generator and the point where 
the ground is located. If now we move switch ^ to contact e, 
which is in direct connection with U, the whole voltage of the 
circuit will act to force a current through the lamp /. From this 
it will be seen that, if the lamp lights up when ^ is on e, we 
know that the ground is on the L side of the circuit, but if the 
lamp lights up with ^ on e' } we know that the ground is on the 
L' side. 

After finding that there is a ground in the circuit, we can 
determine whether it is in the distributing lines or in the gen- 
erator by opening the main switch B, for if upon opening this, 
the lamp / fails to light up, we know at once that the ground is 
beyond B. On the other hand, if opening switch B does not 
affect the lamp /, we know that the ground is in the generator 
or the connections running from it to B. 

If the machine is a motor instead of a generator, we can 
test for ground connections by the same arrangement, but in 
this case the wires d d are to be connected with the line wires 
L V , so that we may be able to test the line for grounds before 
the motor is connected. To connect the wires d d with the line 
wires L L' all that is necessary is to run them to the upper bind- 
ing posts of the main switch B. 

To test the line for ground, the switch B is opened, and then 
switch .? is placed on e and e' in the manner already explained. 
If we find that the line wires are clear, the switch B is closed 
and the test is repeated, and if it shows a ground we know that 
this is located in the motor or in the connections between the mo- 
tor and the main switch B. 

If we now disconnect the field wires of the motor, as is 
illustrated in Fig. 167, and insert a resistance R in the armature 
circuit, we can find whether the ground is in the armature by 
connecting one terminal of a voltmeter with one of the com- 
mutator brushes, and the other with the field frame, or with 
any of the metallic portions of the motor as indicated at c. If 
this test shows the armature to be clear, we disconnect the wires 
from the brushes and connect them with the field terminals and 



250 



GROUNDS AND FIELD SHORT CIRCUITS. 



then repeat the test. If this second test shows that the field 
coils are sound, then we know that the ground is in the connect- 
ing wires. 

Voltmeter V in these tests can be replaced by an incan- 
descent lamp, in the same manner as the lamp in Fig. 166 can 
be replaced by a voltmeter. If a voltmeter is used in either test. 
it should be capable of indicating as high an e. m. f. as that 
of the line current, otherwise the instrument may be seriously 




fig. 167. 

damaged by the current that will flow through it if there is a 
complete ground. 

If a resistance for R in Fig. 167 is not at hand, we can get 
along without it by testing the field coils separately, and then 
disconnecting the connecting wires from the motor and testing 
each one of these independently. In testing these connecting 
wires, we must be careful not to connect their ends, for if we 
do, the main line will be short-circuited the instant switch S 
is closed, and the results may be serious. 

Sometimes a motor or generator may not run well on ac- 



GROUNDS AND FIELD SHORT CIRCUITS. 25I 

count of a ground connection in the field which will allow a 
portion of the current to be diverted from its proper channel. 
If there is a ground in the armature, it is likely to produce 
such a disturbance as to render the machine practically useless, 
and if it is allowed to run, the leakage through the ground will 
soon end in a destructive burn-out, which will require rewind- 
ing the armature. Grounded armatures can seldom be repaired 
before they are burned out, but such is not the case with 
grounded field coils. 

If, without any apparent reason, the brushes begin to spark 
badly, yet are found to be in proper adjustment, we may infer 
that there is some defect in the field coils, either a ground or a 
short circuit. By the method just explained we can determine 
whether there is a ground, and by the process illustrated in 
Fig. 168 we can ascertain whether there is a short circuit. This 
diagram represents a four-pole machine, which may be either a 
motor or a generator. A voltmeter connected with the mains 
L V will indicate the full e. m. f. of the circuit, and if there 
are four field coils, as in the figure, a voltmeter connected with 
the ends c a' of one of the coils, as shown, should show a voltage 
equal to one-quarter of the total. If each coil is tested sepa- 
rately, the one which is short-circuited will show a lower volt- 
age than the others, and in this way we can pick out the defect- 
ive coil. This test is to be made while the machine is running. 
Sometimes, tests of this kind cannot be made with the machine 
in operation. This is generally the case with generators. 

If a generator armature is short-circuited, it can be run 
only a few seconds before it will be burned out. If any of the 
field coils are short-circuited the machine can be run, but the 
sparking at the commutator is liable to be severe. On that 
account the tests for field defects, grounds as well as short cir- 
cuits, are better made with the generator at rest, in which case 
it is necessary to use a battery to provide the testing current, 
and as the voltage of this is not sufficient to give on a voltmeter 
any reading that can be of service, it is necessary to substi- 
tute for the voltmeter a galvanometer; an ordinary detector 
galvanometer will answer the purpose. The most satisfactory 
kind of battery is the dry cell which can be obtained in any 
electrical supply store at a very low cost. 



252 



GROUNDS AND FIELD SHORT CIRCUITS. 



If the galvanometer needle swings around more than 60 
degrees when connected to test a field coil, a resistance should 
be placed in series with it so as to reduce the angle of deflec- 
tion. In making the test in Fig. 168, the battery is connected 
with the mains L V and the galvanometer takes the place of the 
instrument V . The number of cells of battery connected with 




L V will depend upon the resistance of the field coils, and can 
be easily determined by actual trial. If one cell gives a very 
small deflection of the galvanometer needle, say 10 degrees, 
try two cells connected in series, and then three, until the de- 
flection is somewhere between 45 and 60 degrees. If one cell 
gives too great a deflection of the galvanometer needle, use a 
resistance in series with the instrument to cut down the current 



GROUNDS AND FIELD SHORT CIRCUITS. 253 

passing through, thus reducing the deflection of the needle. If 
all the Teld coils are sound, the galvanometer needle will be de- 
flected the same amount when each one is tested, but if one of 
the coils is short-circuited, the deflection of the needle produced 
by it will be smaller. 

If the short circuit does not include the whole coil, the 
reduction in the deflection of the needle will be only a few de- 
grees, but if the short circuit is from end to end of the coil, 
the deflection of the needle will be reduced to nearly nothing; 
thus, by the amount that the deflection of the needle is reduced, 
we can judge as to how much of the coil is short-circuited. 

When the field coil that is short-circuited has been located, 
the next step is to find the defective points. This can generally 
be done because, at the defective points, a sufficient amount 
of heat will be developed to char the insulation and cause it to 
give out the odor of burned shellac. If the damage cannot be 
repaired without defacing the coil, which will most likely be 
the case, as the short-circuited points are almost sure to be 
below the surface, then rewinding is the only proper remedy. 

Temporary repair can be made by removing the wire from 
a portion of the coil, as is illustrated in Fig. 169, holding the 
rest in position by means of wooden blocks. As each layer is 
removed, the ends of the wires on both sides of the opening are 
tested, and when the layers that are short-circuited are reached, 
the test will show that they are connected with each other — 
that is, if one of the ends of the wires from the galvanometer 
is connected with the end of one layer or wire on the coil, and 
the other end is connected with another layer, and the needle 
moves, then we know that these two layers of wire are short- 
circuited. 

After all the short-circuited layers have been picked out 
in this way, the perfect layers can be reconnected, being careful 
to connect the ends that wind right sided with those that wind 
left sided ; and also being careful that all the layers are con- 
nected in series. This latter result can be accomplished by con- 
necting one of the wires from the galvanometer with the end 
of the top layer of the coil ; then with the other end of the 
galvanometer wire, the other end of the top layer can be found. 
This is to be connected with any end that winds in the opposite 



254 



GROUNDS AND FIELD SHORT CIRCUITS. 



direction, and the remaining end of this second layer can then 
be picked out by the aid of the galvanometer, in precisely the 
same way that the remaining end of the top layer was found. 
This end is in turn connected with another end that winds in 





fig. 169. 



the opposite direction, and thus the connecting is carried on 
until all the layers that are perfect are joined up. 

The machine will not run perfectly when patched up in 
this way, but unless a large number of turns have been ren- 
dered useless by the short circuit, it will work well enough for 
temporary use, until a permanent repair can be made. 



REPAIRING SHORT CIRCUITS IN ARMATURES. 255 



CHAPTER XXXIV. 
Repairing Short Circuits in Armatures. 

DIRECTIONS for finding short circuits in the armatures 
of generators are not necessary, as in almost every case 
they find themselves ; and the first notice we get of the 
fact is that the machine gives off a very strong smell of burn- 
ing shellac, which is immediately followed by smoke and, pos- 
sibly, some flame. After this the generator is useless until the 
armature is rewound. In some cases, the short circuit is only 
partial, and then the only way that its presence can be detected 
is by the odor peculiar to hot shellac. This is a condition that 
is seldom encountered, for even if the short circuit is imperfect 
at the start, when it reaches the point where the armature be- 
gins to heat up, it progresses so rapidly that, before we know 
what has occurred, the wire is burned out. 

When a short circuit forms in the armature of a generator, 
it affords a path of comparatively low resistance through which 
a portion or all of the current can circulate, according to the 
position of the points between which the short circuit is ef- 
fected. If the contact at these points, between the metallic 
parts of the circuit — that is, between the bare wires — is not very 
good, the resistance may be so high as to permit only a small 
current to pass ; but this current will heat up the points of 
contact, and as a rule will result in making the connection 
more perfect, either by charring the small amount of insulating 
material between the wires, or by expanding the metal until the 
two parts come into more perfect contact. 

Whichever way the action may proceed, the result will be 
that the resistance in the short circuit path will be reduced and 
the current increased, and as the action progresses, the change 
in resistance and current strength becomes more rapid, until 
a point is reached where the heat generated is enough to make 
the shellac smell; only a few seconds more will be required to 
develop sufficient heat to burn the insulation and perhaps fuse 
the wire. Thus it will be seen that in generators, short circuits 



256 REPAIRING SHORT CIRCUITS IN ARMATURES. « 

come almost without warning, and it is almost never that warn- 
ing is given in time to save the armature from destruction. 

With motors, however, the case is quite different. As a 
rule, if a motor armature is short-circuited it will not rotate 
when the current is turned on, even if the machine is running 
light. If it is helped by hand, it may rotate slowly, but with 
an irregular, jerky motion. In most cases, however, when 
turned b} r hand it will make a portion of a revolution and will 
then come to a standstill. In order to move it from the posi- 
tion in which it stops, a considerable effort will be required ; 
but as soon as it has been carried beyond a certain point it will 
immediately swing forward of its own accord, and again come 
to a stop at the first position. 

If short-circuited, a motor armature will not be burned out 
because it cannot rotate, since there is no electromotive force 
other than that of the supply circuit to force a current through 
the short circuit, and the supply current is controlled by the 
resistance of the starting box and the safety fuses or circuit 
breakers, whichever may be used, so that it cannot rise above 
a safe strength. 

It is not a difficult matter to find the position of the short- 
circuited coils in a motor armature, but to find the exact posi- 
tion of the points of contact is, in most cases, rather difficult 
without removing some of the wire. By the aid of the accom- 
panying diagrams we can illustrate the means that may be em- 
ployed for locating short circuits. 

In Fig. 170 the circle C represents the commutator of a 
motor armature and V is a voltmeter. This diagram represents 
a two-pole machine, for which two commutator brushes are 
required. The current enters through the upper brush and 
passes out through the lower one. From the segment of the 
commutator on which the upper brush rests, the current passes 
in two circuits through the armature coils until it reaches the 
segment on which the lower brush rests. After passing 
through each armature coil, the current reaches the wire that 
connects with the corresponding commutator segment, so that 
we may say that these connecting wires are reached progres- 
sively on each side of the commutator, in the manner indicated 
by the arrowheads on circle C. 



REPAIRING SHORT CIRCUITS IN ARMATURES. 



257 



Now, to force the current through the armature wire re- 
quires a certain electromotive force. Suppose that the armature 
is held so that it cannot rotate, and that one wire from the 
voltmeter V is connected with the upper brush, while the other 
wire is connected at different points on the surface of the com- 
mutator, as indicated at c. If the point of contact c is near to 
the upper brush, say the width of one segment, then the voltage 
indicated by the voltmeter V will be that required to force the 
current through one of the armature coils. If the point c is 
now advanced to the second segment, the voltmeter will indicate 
the voltage required to force the current through two armature 




FIG. 170. 



FIG. 171. 



coils. In the same way, if the point c is advanced to the third 
segment, the voltmeter will indicate the voltage required to 
force the current through three coils, of the armature wire. 

If we draw a diagram such as Fig. 171, which consists of 
a circle, A, and a number of radial lines, 1, 2, 3, 4, etc., equal to 
the number of segments in the commutator ; and if on these 
lines we mark off distances extending outwardly from the circle, 
equal to the voltage indicated in the instrument V with the 
point c in the corresponding position ; then, by tracing through 
the marks so obtained a curve, B, we shall have a representa- 
tion on paper of the manner in which the voltage rises, as the 



258 



REPAIRING SHORT CIRCUITS IN ARMATURES. 



point c in Fig. 170 is advanced from the upper brush toward 
the lower one. 

This curve will show us the voltage required to force a 
given current through the armature wire from the point where 
the upper brush connects with it to the point where c makes 
contact. If the armature is not short-circuited at any point, the 
resistance of all the coils will be practically equal, and as the 
voltage required to force a current through a resistance is equal 
to the current strength multiplied by the resistance, it follows 
that, as the resistance is increased uniformly by adding coil after 
coil to the circuit between the upper brush and the contact 
point c, the voltage will also rise uniformly. 




172. 



For making this test it is necessary to insert a large resist- 
ance in the armature circuit, so as to keep the current down to 
a safe limit. The motor starting box is of sufficient resistance, 
but it cannot be used for the purpose because the resistance 
coils are not of sufficient size to be kept in the circuit for more 
than a few seconds. The voltmeter used should be of capacity 
to indicate small voltages. It is not always possible to obtain 
a resistance suitable to be placed in the armature circuit, and 
likewise it is not always convenient to obtain a low-reading 
voltmeter — one that will indicate from 10 volts downward. We 
will, therefore, explain how this tee't can be made with a gal- 
vanometer. 

For this purpose are required one or two dry battery cells, 
(which can be obtained in any electrical supply store at a cost 



REPAIRING SHORT CIRCUITS IN ARMATURES. 



259 



of 25 or 50 cents), a galvanometer of any kind, and a resistance, 
R, to place in the galvanometer circuit, as shown in Fig. 172. 
The resistance R is required because a very small current will 
produce a decided deflection of a galvanometer needle. The 
simplest form of galvanometer is known as a detector galvano- 
meter, and good ones can be obtained for $2 to $3. 

To test the armature with a galvanometer so as to obtain 
the curve B of Fig. 171, connect the two brushes with the 
terminals of the dry battery ; -then connect one terminal of the 
galvanometer with brush b, and the other terminal through re- 




fig. 173- 



sistance, R, and the sliding contact c with the lower brush. Ad- 
just the resistance R so that the galvanometer needle is de- 
flected about 60 degrees, then move the sliding contact c back, 
segment by segment, and mark down, on a diagram prepared 
like Fig. 171, the degrees of deflection for each position of the 
contact c. 

In this way a curve can be obtained which shows how the 
resistance varies from point to point between the brushes. It 
does not tell us the voltage required to force a given current 
through the wire, as does the test with the voltmeter, but that 
makes no particular difference. It may be well to mention that 
in using a galvanometer the instrument must be set level so 



200 



REPAIRING SHORT CIRCUITS IN ARMATURES. 



that the needle will swing freely, and also that it must be so 
placed that the needle points directly to the zero mark when 
there is no current passing through the instrument. In making 
a test with the voltmeter, as in Fig. 170, the armature is con- 
nected with the circuit in the manner shown in Fig. 173, with 
a resistance R sufficiently large to keep the current down to 
about the full-load strength. 

In Fig. 171 the curve shown in regular like that for a per- 
fect armature. It is not in correct proportion for such an arma- 
ture, but it indicates the way that a test curve of a perfect 
armature would look. 




fig. 174. 



FIG. 175. 



Now suppose that we test a short-circuited armature. Let 
the points that are short-circuited be located at a a, Fig. 174. 
Then in starting the curve, with the upper brush at 0, the curve 
obtained would rise until it reached line 2 and the next meas- 
urement at position 3 would show but little rise in the curve. 
The voltage will be nearly the same until line 6 is reached. This 
shows at once that at c there is a direct connection with some 
point in the wire near line 2 which cuts out a large portion of 
the resistance. 

To find out just where this point is, we revolve the arma- 
ture one or two segments, and then test for another curve. Sup- 
pose that after several trials we obtain a curve such as shown 



REPAIRING SHORT CIRCUITS IN ARMATURES. 26l 

in Fig. 175, which rises hardly any until line 4 is reached, from 
this drop in the curve we realize at once that the segment at c 
is in a direct contact with the one from which we started the 
curve, for between these two points, the curve indicates prac- 
tically no resistance; hence the short-circuited points a a are 
located one at segment c and the other at the segment at the 
top of the figure, on the line 0. 

If, in Figs 174 and 175, we were to continue to test the curve 
all the way around to the lower brush, we should obtain curves 
that would rise in a uniform manner as shown in these diagrams ; 
provided there were no other short circuits in the armature; 
but if there were other short circuits, then for each one of these 
there would be a flat in the curve. 

Sometimes an armature is short-circuited in several places; 
therefore, in making a test it is always advisable to obtain read- 
ings of the instrument for every position between the two 
brushes. If more than one short circuit is found, the segments 
with which they are all connected can be located by turning the 
armature around, one segment at a time, and making a test in 
each position so as to find those between which the rise in the 
curve is zero, as in Fig. 175 from a to c. 

In making the foregoing test with a multipolar armature, 
the readings are taken for the number of commutator segments 
between two adjoining brushes, and the armature is advanced 
segment by segment and new readings taken to locate the short- 
circuited points. If the armature is parallel connected, the pre- 
cise segments with which the short-circuited coils are connected 
can be located ; but if the armature is series connected, the best 
we can do is to find the several segments that connect with the 
short-circuited coils. In a four-pole armature, there will be two 
segments that appear to be connected with each short-circuited 
point, if the armature is series wound ; and in a six-pole arma- 
ture there will be three segments apparently connected with each 
short-circuited point. By making a careful test the one of these 
segments that is the nearest to the point can be determined, as 
the others will give readings a trifle higher. 

After the short-circuited points are located within certain 
armature coils, the next step is to see whether, by inspecting 
these coils, we can find the defective points. If the armature 



262 REPAIRING SHORT CIRCUITS IN ARMATURES. 

coils are held in place by means of wire bands, we may expect 
to find the short circuit formed through one of these. If no 
defects can be found at these points, then we must endeavor to 
determine whether the coils cross each other at the ends of the 
armature and, if possible, ascertain whether the defect is located 
at these points. If we find that there is no defect at these cross- 
ings, then the only place in which it can be found is between 
the armature coils and the armature core, and both defective 
coils must be in contact with the iron core. 

In some cases it is possible to find the short-circuited points 
without removing wire from the armature; therefore, in every 
case, effort should be made to locate the difficulty without un- 
winding the armature. If, at last, we find that the wire must 
be removed, we should start from points that will enable us to 
reach the short circuits by removing the smallest possible amount 
of wire. When the defect is uncovered, it may be found that it 
can be remedied by simply inserting a small piece of insulating 
material and without using new coils. When the defect can be 
found from an external inspection, in most instances the short 
circuit can be easily removed by slipping between the points in 
contact a sheet of some stiff insulating material. In most cases, 
a piece no larger than a postage stamp will be all that is re- 
quired. 



FINDING AND REPAIRING BROKEN WIRES IN ARMATURES. 263 



CHAPTER XXXV. 
Finding and Repairing Broken Wires in Armatures. 

BROKEN wires, or co speak more correctly, open circuits 
in an armature, are far more common in small machines 
than in large ones. On that account they are more often 
met with in motors than in generators, because the former are 
more common in the smaller sizes. The reason for more trouble 
with small machines is simply that the armature wire is smaller, 
hence more easily broken. 

Broken wires proper are generally due to vibration produced 
while the armature is in motion. In some cases they may be 
due to defects in the wire which are not noticeable when the 
armature is being constructed, but such is not often the case. 
For one reason or another, there may be a flaw in the wire, 
and this will in time be developed into an actual fracture by 
the contraction and expansion due to the heating and cooling 
of the armature. 

In ninety-nine cases out of a hundred, it can be assumed 
that the break is not due to a defect in the wire, but to the 
continual vibration to which it is subjected when the machine 
is running. The portion of the wire that is wound tightly 
against the armature core, cannot vibrate as much as that which 
is held loosely; hence, the proper places in which to look for 
breaks are in the portions of the wire that are held the least 
firmly. Of all these parts, the connections running from the 
armature coils to the commutator segments are the ones having 
the least support. Experience shows that in almost every case 
a broken wire will be found to be located in these connections, 
or directly adjoining them. 

For breaks the most common place is at the point where 
the connection is made with the commutator segment. In some 
cases the wire will be found broken off at this junction, but 
more often the connection will be simply loose. In some ma- 
chines these connections are made by means of screws, and in 
others the wire is soldered into the segments. Screw connec- 
tions are quite liable to become loose, especially if the screw 



264 FINDING AND REPAIRING BROKEN WIRES IN ARMATURES. 

presses directly against the wire, as is sometimes the case. If 
the wire is held between the end of the segment and a clamping 
cap, by means of two screws, there is less liability of the con- 
nection coming loose. Soldered joints, however, are the most 
reliable, if properly made, and are more generally used. 

One advantage claimed for the screw connection is that, if 
the armature has to be disconnected from the commutator, it 
can be done with loss trouble than with soldered connections. 
This advantage, however, is not of much account, if the ma- 
chine is properly made, because it is only in case of a break- 
down that the commutator has to be removed. 

. If there is a broken wire or connection in the armature of 
a motor, the machine will continue to run, but the severe spark- 
ing at the brushes will show to the attendant that something is 
out of order. In a generator of the two-pole type, a broken wire 
will stop the generation of current, but in a multipolar genera- 
tor, a broken wire will not, as a rule, do so. As already stated, 
the presence of a broken wire in the armature of a motor can 
be detected by the sparking at the commutator brushes, which 
is also true* with respect to multipolar generators. The spark 
produced by broken wires is of such a character that it can be 
easily detected by any one who has seen it before. When it is 
understood how a break in the armature circuit affects the 
operation of the machine, the appearance of the spark can be 
readily pictured in the mind's eye. 

On an armature the wire is so connected as to form an 
endless loop, and the brushes are placed upon the commutator 
so as to make connection with this loop at points that divide it 
into two equal parts, provided the machine is of the two-pole 
type. For a four-pole armature there would be four brushes, 
and these would divide the endless loop into four equal parts, 
and similarly a six-pole machine would have six brushes, that 
would divide the wire into six equal parts. The commutator 
is simply a sliding contact arrangement by means of which the 
connection between the brush and the armature wire may be 
shifted along as the latter revolves. 

Commutator segments are connected with the ends of ad- 
joining armature coils, so that when one segment slides under 
the brush and the next one behind it comes into contact, the 



FINDING AND REPAIRING BROKEN WIRES IN ARMATURES. 265 

connection with the armature wire is shifted ahead the length 
of one coil 

If the armature wire is perfect — that is, without a break — 
the current passing in through the upper brush will divide into 
two equal parts, and one-half will flow through the one side and 
the other half through the other side of the winding; these halves 
will meet at the lower brush. Suppose, however, that there is a 
break in the wire, as indicated at b in Fig. 176. Then it is evi- 
dent that the only path by which the current can reach the lower 
brush is through side A. If the current flowing through the 
armature has a sufficiently high voltage, it will be able to jump 
over the break at b, as indicated by the line a, and thus estab- 
lish a path through the B side of the wire. 




fig. 176. 

In an electric motor, this action actually takes place, and a 
spark leaps out of the end of the brush, as shown in Fig. 177 
at a. If the motor is operated by a current of low voltage, say 
no, the spark a may not draw more than ^ or ^ inch, but 
with higher voltage it may lengthen out to 2 inches. In some 
cases, when the segments between which the break is located 
pass to some distance beyond the brush, the spark jumps from 
one segment to the other across the insulation, giving the ap- 
pearance of a somewhat transparent ring of flame all the way 
around the commutator. 

One most striking peculiarity of the spark due to a broken 
wire is its flickering in time with the rotation of the machine. 



266 FINDING AND REPAIRING BROKEN WIRES IN ARMATURES. 

Each time the segments between which the break is located pass 
under the brush, the spark draws out, until the distance be- 
comes so great that it breaks. As this drawing-out process is 
repeated at each revolution it causes tne spark to nicker and this 
is accompanied by an intermittent noise, the noise and spark 
keeping time with the rotation of the armature. 

If the break occurs in the armature of a two-pole generator, 
the machine will not generate, because the armature itself must 
supply the voltage that drives the current through the circuit, 
and as there is a break when it reaches the position of b in Fig. 
176, the current will not bridge it, for the simple reason that 
the armature does not get a chance to build up a sufficient 




fig. 177. 



fig. 178. 



voltage, on account of the break. If the generator is of the 
four, six or eight-poie type, it will generate, because then the 
broken wire at b disables only one-quarter, one-sixth or one- 
eighth of the wire, and the remainder is sufficient to develop the 
necessary voltage to force the current over the break. 

If an armature in which there is an open circuit or broken 
wire is run for a few seconds and then stopped, it will be found, 
upon examining the commutator, that in the case of a two-pole 
machine there will be one segment which has a corner badly 
burned away, as shown in Fig. 178. The segment diametrically 
opposite to this one may also show a slight burning, but nothing 
like as much as the one at a. If the machine is of the multipolar 
type there will be as many segments burned as there are pairs 
of poles, and these will be equally spaced all the way around 



FINDING AND REPAIRING BROKEN WIRES IN ARMATURES. 267 



the circle. One of these, however, will be found to be burned 
more than the others, and to this one and to the segment back 
of it are connected the ends of the broken wire. 

As already stated, if the machine is a two-pole generator, 
it will not generate with a broken wire in the armature, but 
from considering the action explained in connection with Fig. 
176, it will be seen that if we could form the connection indi- 
cated in that figure by the line a, a current could be obtained, 
and such is actually the case. The simplest way of making this 
test is illustrated in Fig. 179, in which a strip of metal a is 
shown resting against the brush holder D with the end bearing 
upon the face of the commutator C. If the strip a is bent so that 




fig. 179. 



FIG. l8o. 



it can be made to bear upon the commutator some distance 
ahead of the brush, the machine will generate as long as the 
strip is in position. By running the armature for a few min- 
utes with the strip in place, the segments connected with the 
defective wire will be burned and then, upon stopping the ma- 
chine, the broken wire can be located. 

Generally, when the broken wire has been located by the 
process explained, the disconnected ends can be easily found. 
In most cases the break will be simply a loose connection between 
the wire and the commutator segment. If this is not the cause 
of the break, the wire may be broken off just where it passes 
out of the shank of the segment. In either of these cases, the 
break can be easily repaired with a soldering iron. In some 



268 FINDING AND REPAIRING BROKEN WIRES IN ARMATURES. 

cases, however, the broken ends cannot be found, and then the 
only remedy, short of disconnecting the armature and removing 
the wire until the ends are found, is to bridge the break, which 
is accomplished by simply making a connection between the seg- 
ment a of Fig. 178 and the one back of it ; that is, between the 
burned segment and the one back of it. This connection can 
be made by soldering a strip of brass to the two shanks, as 
shown at a, Fig. 180. Whenever this method of doctoring up 
the armature is resorted to, it is advisable to remove from the 
two connected segment shanks the ends of the coil. in which the 
break is located; for it is possible for the break to be of such 
a character that it will mend itself, temporarily, when the ma- 
chine is running. If it should, as the patch a forms a short 
circuit, the current developed in the coil would be very strong, 
and might heat the wire to such an extent as to damage the 
insulation of the adjoining coils. 

This method of curing a broken wire, when the end of the 
break cannot be found, must be regarded as only a temporary 
expedient, and, as soon as possible, the armature should be 
taken out of the machine and, if necessary, the wire should be 
removed until the break is found and then repaired in a work- 
manlike manner. An armature doctored up in this way will 
run for any length of time and will continue to run even if a 
large number of breaks are bridged in the same way. In fact, 
the writer has seen armatures running with more than one-quar- 
ter of the commutator segments bridged, but the fact that the 
machine will run in this way does not prove that it is in perfect 
condition. As a matter of fact, it is far from it. 



CONNECTION OF SHUNT-WOUND MOTORS WITH SUPPLY WIRES. 269 



CHAPTER XXXVI. 
Connection of Shi t >:t-wound Motors with the Supply Wires. 

TO MAKE these connections the proper way is shown in 
Fig. 181, in which L L' are the supply wires, A the motor 
armature and M the motor field magnet coils. At R 
is located a resistance, with a switch arranged to cut it out of the 
circuit, commonly called a motor starter. At B is placed a two- 




fig. 181. 



pole main knife switch and just beyond this safety fuses / are 
located. 

One thing that may perplex the novice is, that while there 
are only two line wires L L' there are three wires, or binding 
posts, on the motor with which connections must be made, and 
there are also three binding posts on the motor starter R. An 
investigation of the motor will at once show that of the three 
wires, two come from the commutator brushes, and one from 
one end of the field coils. If the novice has good eyes, he will 



270 CONNECTION OF SHUNT- WOUND MOTORS WITH SUPPLY WIRES- 

soon discover that the other end of the field wire runs to one of 
the commutator brushes, as shown at b in the diagram. 

It is clear that, if the three motor binding posts are all con- 
nected with the terminals of the motor starter R, there will be 
no way of making connections with the main switch B ; there- 
fore, all the motor wires cannot be connected with starting box 
connections. 

Fig. 181 shows the proper connections for the type of mo- 
tors commonly used to drive machinery, and which are operated 
by current derived from circuits that feed incandescent lights. 
This type is technically called a constant potential, shunt-wound 
motor. It is called constant potential because it is so designed 
that it will operate properly when supplied with a current of 
constant electromotive force— that is, a current whose voltage 
does not vary more than 3 per cent. It is called a shunt-wound 
motor, because the current that passes through the field mag- 
netizing coils M is shunted from the main current, which passes 
through the armature. 

In the diagram it can be seen that if the switch S, of the 
motor starter R, is turned to the left, so as to make connection 
with the first contact of R, the current can pass through the 
loops r r to the lower commutator brush and thus through the 
armature to the upper brush, whence it returns to the supply 
line. At the same time a separate current can flow from the 
first contact of R through the lower wire of the field coils M, 
and thus reach the main current at the upper brush b. From 
this it will be seen that the current that passes through the field 
coils M is shunted from the main line, so to speak, at the starting 
box R, and joins the line again at the upper commutator 
brush b. 

For the field coils, the wire is fine, and of great length, and 
its resistance is so high that only a small amount of current can 
pass through it, the amount ranging from 5 per cent of the total 
in small motors down to Ij4 per cent in large ones. The arma- 
ture wire, on the other hand, is made quite large, so as to carry a 
large current without being overheated. In addition to being 
large, it is comparatively short, so that its resistance is very low — 
that is, it impedes the passage of the current to but a slight 
extent. 



CONNECTION OF SHUNT-WOUND MOTORS WITH SUPPLY WIRES. 271 

If the armature were held so that it could not revolve, and 
the two commutator brushes were connected directly with the 
wires from the main switch B, an excessive current would pass 
through the armature, possibly twenty or thirty times as strong 
as that required to develop the full power of the motor; this 
current would soon destroy the armature. When the armature 
rotates, however, there is a back pressure developed in its wind- 
ing, which is called a counterelectromotive force, and acts to 
hold back the current, thus preventing it from increasing to an 
excessive value. The faster the armature revolves, the higher 
will be the back pressure. 

In the starting box R, the loops rr are resistances, gener- 
ally made of wire wound in the form of spiral springs. This 
resistance impedes the flow of current. When the motor is 
started, switch 6" is moved to the first contact of the motor 
starter, and then the current that passes to the armature has to 
flow through all the resistance loops rr, and thus is cut down 
to the proper strength. As soon as the armature begins to re- 
volve, it develops a back pressure, and as this acts to cut down 
the current strength, it can replace the resistance in the starter 
R. As the speed increases, switch S is moved from contact to 
contact, and by the time the armature has attained its full 
velocity, all the resistance of R will be cut out — that is, switch 5 
will be advanced to the last contact. 

From the foregoing, it will be seen that the object of the 
motor starting box ir, to provide a resistance that can be in- 
serted in the armature circuit, while starting, so as to keep the 
current strength down to a proper limit while the speed and back 
pressure are building up to their normal running values. 

Safety fuses f are provided to protect the armature from 
the effects of excessive currents at any time. In starting, if 
switch 5" is advanced too fast, the current will increase too fast, 
as the back pressure developed will be insufficient to replace the 
resistance cut out of the series of loops r r. Safety fuses melt 
when the current is too great, and thus open the circuit; they do 
not give way, however, the instant a strong current begins to 
flow, since sufficient time must pass for the metal of which they 
are made to be heated to the melting point. 

As it is desirable in most cases to provide a protective de- 



272 CONNECTION OF SHUNT-WOUND MOTORS WITH SUPPLY WIRES. 

vice that will act instantly, when the current rises suddenly to 
very great strength, magnetic cutouts are also provided. Th^se 
are sometimes independent pieces of apparatus, and are called 
circuit breakers; and in some cases they form part of the 
motor starter. The latter is then called an automatic overload 
starter. 

It sometimes happens that, when a motor is running, the 
current in the supply main L V for some reason dies out, and 
the machine comes to a standstill. In every such case, the 
starting box switch 6* should be opened; for, if not, when the 
current is re-established, the armature will be connected in the 
circuit with all the resistance of R cut out, and being at a 
standstill, the current passing through it will rise to a danger- 
ous strength, as already explained. To prevent this contingency, 
starting boxes are made so that they will throw the switch S 
to the open position when the current dies out. Such boxes 
are called underload, or "no voltage" motor starters. Boxes 
are also provided with both kinds of safety devices — the over- 
load and the underload. 

Safety fuses are proportioned so that they will be melted 
with a current about 50 per cent stronger than the full-load cur- 
rent, provided this continues for a considerable length of time, 
say 5 minutes. The magnetic cutout is set so that it will not 
act with as weak a current as that, but when it is set for cur- 
rent of, say, double the normal strength, it will act instantly, 
if the current reaches this magnitude. Thus it will be seen 
that, if safety fuses and magnetic cutouts are both provided, 
the first are used to protect the armature from injury due to a 
prolonged current of about 50 per cent more than the full-load 
strength, while the latter are set to protect the machine from 
a sudden increase of much greater magnitude, or from a total 
suspension of the current. 

Connections shown in Fig. 181 are the most desirable, but 
in many old-style motor starting boxes, and in some of modern 
make for small motors, the connections are made as in Fig. 182. 
The difference between the two is that in Fig. 181 the arma- 
ture and the field coils are connected so as to form a closed 
circuit at all times, even when the switch ^ is in the open 
position, as shown. 



CONNECTION OF SHUNT-WOUND MOTORS WITH SUPPLY WIRES. 273 

In Fig. 182, when switch S is in the open position, the 
circuit between the field and armature is open. This is ob- 
jectionable, because, if the field coil circuit is opened, there 
will be a heavy spark at the end of the contact a, and, in addi- 
tion, there is danger of the insulation of the field coils being 
punctured. When wire is wound in coils of many turns, as is 
the case with the field coils of shunt motors, a very high volt- 
age is developed at the instant the circuit is broken. This 
voltage is commonly called the kick of the coil. If a motor 
is of small capacity and for low voltage, say I horsepower and 
no volts, the kick may not be strong enough to damage the 
insulation, but it will produce a sufficient spark at the switch 
to roughen the contacts. With a larger motor of higher volt- 




fig. 182. 



age, say 20 horsepower and 220 to 500 volts, the kick of the 
field coils will be so strong, if the circuit is opened, as to be 
almost sure to puncture the insulation. On this account motor 
starters should always be connected as in Fig. 181. 

One objection to the connection of Fig. 181 is that when 
the motor is running, the field current has to pass through all 
the resistance loops r r of the starter. This objection, however, 
is far from being serious, because the resistance of these loops 
is small in comparison with that of the field coils, and it reduces 
the strength of the field current by an amount almost too small 
to be noticed. Some makers of starting boxes provide a plate 
contact, as shown at a, Fig. 182, for the purpose of letting the 
field current flow directly to the field coils without passing 



274 CONNECTION OF SHUNT-WOUND MOTORS WITH SUPPLY WIRES. 



through the resistance r r. To accomplish this result the plate 
is connected as shown dotted at c in Fig. 182. As will be 
seen, with these connections the field current flows through 
the arc a to the connection at c, Fig. 182, no matter where the 
switch S may be, between c and the other end. 

Sometimes it is desired to connect a shunt motor so that 
it may be run in either direction. To accomplish this all that 
is necessary is to provide means whereby the armature current 
may be reversed; if the field current is also reversed, the motor 
will run in the same direction as before. To reverse a motor, 
a reversing switch must be used, as shown at D in Fig. 183. 
L 




FIG. 183. 



In this diagram, the main switch B and the fuses f of Fig. 181 
are omitted for the sake of simplicity. The motor starter is 
placed at R. Most types of automatic overload and underload 
starters can be used with reversing motors, as they are not 
affected by the direction of the current through their magnet 
coils. 

In Fig. 183 it will be seen that with the reversing switch D 
in the position shown, the current from the upper line wire 
passes to the lower commutator brush, and then through the 
armature and the motor starter, to contact b, and to the lower 
line. The field coil current is shunted from the points d d. 
With this arrangement, when the motor is stopped, by open- 



CONNECTION OF SHUNT-WOUND MOTORS WITH SUPPLY WIRES. 275 

ing the reversing switch or the starting box switch, the field 
coil circuit is not opened, as points d d are not disconnected, but 
the field coils are not disconnected from the line. It is difficult 
to make a reversing .switch that will not open the field circuit, 
yet will break the line connection. 

One way in which a reversing switch can be made to pre- 
/ent this difficulty, of opening the field circuit, is shown in 
Fig. 184. In this diagram the field coil terminals run to the 
contacts e, e' and f, which are connected with contacts b, b' and 
c respectively, by the blades of the reversing switch D. The 




reversing switch is shown in the open position and, as will be 
noticed, the field coils are disconnected from the main line, but 
at the same time the field coil circuit is broken, so that the 
injurious effects produced by the kick of the coils will be ex- 
perienced. 

To get around this trouble, it is common practice to con- 
nect a number of incandescent lamps in parallel with the field 
coils, as indicated in the diagram by the three circles below the 
field. An objection to this plan is that, when the motor is run- 
ning, some current flows through the lamps, and this causes 
just so much loss; but by increasing the number of lamps con- 
nected in series, the current can be cut down to a small amount. 



276 CONNECTION OF SHUNT-WOUND MOTORS WITH SUPPLY WIRES. 

There are ways in which the reversing switch can be made 
so that when the motor is running, the lamp circuit, shunted 
from points d d, will be open, and will only be closed just be- 
fore the reversing switch D is opened. These constructions, 
are rather complicated and are hardly necessary, since the current 
that will pass through the lamp circuit around the held coils can 
be made so small as to amount to practically nothing. 



CHANGING THE SPEED OF MOTORS. 277 



CHAPTER XXXVII. 
Changing the Speed of Motors. 

MOTORS are manufactured so that they may run at a 
constant velocity or so that the speed may decrease as 
the load increases, or again so that the speed may be 
changed at will by means of a hand regulator. 

That known as a series-wound motor is the simplest form. 
A motor of this type is illustrated diagrammatically in Fig. 185, 
in which A represents the armature, C the commutator and M 
the field magnet coils. The diagram also shows the way in 
which such a motor is connected with the circuit, L L' being 
the line wires, B a main switch for making or breaking the 
line connections, and R a rheostat which is used to start the 
motor. This type of machine is called a series motor, because 
the armature and the field coil windings are connected in series 
with each other, so that all the current that passes through the 
field coils also passes through the armature. As shown by the 
arrow heads in the diagram, the current first passes through 
the field coils and then through the armature. 

This type of motor has a natural tendency to run fast when 
the load is light, and slow when the load is heavy. If the belt 
is thrown off, it will run away, and as it is loaded down, it will 
continually reduce its speed. If the load is increased without 
limit, and there is no circuit breaker or safety fuse to open 
the circuit, the motor will keep on reducing its speed until the 
current becomes so strong as to heat the wires sufficiently to 
burn the insulation, and thus destroy the machine. From this 
it will be seen that a series motor will not run at a constant 
speed unless the load is constant ; with a varying load, the 
speed will vary. 

There is no way in which a series motor can be made to 
run at a constant speed with a varying load ; hence, if you have 
a machine of this kind and want it run at a constant velocity 
with variable load, make up your mind that it cannot be made 
to do it. Series motors are used principally to run trolley cars 



278 



CHANGING THE SPEED OF MOTORS. 



and, to some extent, for operating hoisting machines, pumps 
and fans. 

Although a series motor changes its speed with changes in 
the load, the rate at which it changes its speed may not always 
be just what is required. By means of what is commonly called 
a motor controller, the speed can be changed by hand in any 
manner desired, within certain limits. A motor controller is 
constructed in substantially the same way as a motor starter, 
that is, it consists of a resistance and a contact lever, the two 
being connected so that more or less of the resistance may be 
cut into the motor circuit by the movement of the lever. The 
difference between a motor starter and a controller is one of 




fig. 185. 



size only; the starter has to remain in circuit only a few sec- 
onds while starting the motor, and on that account the resist- 
ance can be made of small wire. The controller may have to 
remain in circuit for a long time; therefore, the resistance must 
be made of wire of such size that it can carry the full load 
current continuously without becoming overheated. 

In Fig. 185, R may be taken to represent a motor starter or 
a motor controller, the loops r r representing the resistance that 
is cut in and out of the motor circuit. When the switch .S" is 
placed on the first contact to the left, the current entering at 
contact b will have to traverse all the resistance loops r r; but 



CHANGING THE SPEED OF MOTORS. 



279 



when ^ is advanced to the contact b, the current can pass di- 
rectly to the end of the field coils without passing through any 
of the resistance loops r r. 

Motor controllers can' be used as motor starters, but a 
motor starter cannot be used as a controller, simply because 
it is of too small capacity. If in Fig. 185, R is a controller, then 
it is evident that by the movement of the switch ^ by hand to 
any position, any number of the resistance loops r r can be cut 




FIG. l86. 



into or out of the motor circuit; hence, no matter whether the 
load be light or heavy, the speed of the motor can be varied 
by the movement of the controller switch 5". The highest 
speed that the motor can attain will be with the switch S rest- 
ing upon contact b, and the lowest will be with the switch rest- 
ing on the first contact at the left-hand side. Thus the speed 
regulation is limited to a certain range, which is made large 
or small by increasing o** decreasing the resistance of the con- 
troller R. 

Another way in which the speed of a series motor can be 



280 CHANGING THE SPEED OF MOTORS. 

varied at will is by providing a circuit around the armature, 
which might be called a bypass circuit. Such an arrangement 
is illustrated in Fig. 186. In this diagram it can be seen that 
when the current reaches point a it can split, part going through 
the motor by way of wire b, and part around the armature by 
wire c. In the bypass circuit there is a resistance R' , and a 
speed controlling resistance R. The first-named resistance is 
made of such value that, when the controller switch S' is in 
the position shown and all of the resistance R is out of the cir- 
cuit, the current flowing through the bypass is not more than 
the field coils M can carry, in addition to that coming from the 
armature. With this position of the switch, the current di- 
verted from the armature is the greatest, and the speed the 
lowest. By moving the switch S r to the right, additional re- 
sistance is cut into the bypass and thus more current is forced 
through the armature, and the speed is increased, for the same 
load. 

This means of controlling the speed of series motors by 
hand has the objection that all the energy used up in the by- 
pass represents so much loss. It is much like varying the speed 
of an engine by opening a connection between the live steam 
pipe and the exhaust. 

By connecting the field coils in parallel, as illustrated in 
Fig. 187 the natural speed of a series motor can be increased. 
Another way to increase the speed is by using a bypass circuit 
around the field coils, as shown in Fig. 188. This arrangement 
has the objection of wasting current the same as that shown 
in Fig. 186; but it is much more economical because the loss in 
the bypass is only a small fraction of the total energy used by 
the motor. The loss in the arrangement of Fig. 186 is prob- 
ably ten times as great as in that of Fig. 188. The advantage 
of Fig. 188 over Fig. 187 is that, by making the resistance R 
in the bypass circuit in adjustable form, — that is, like a con- 
troller — the increase in speed of the motor can be made greater 
or less, as may be desired ; while by the coupling of the field 
coils, in parallel only one change in speed can be obtained. Just 
how much the speed will be increased by the arrangement of 
Fig. 187 cannot be determined accurately without knowing all 



CHANGING THE SPEED OF 'MOTORS. 281 

the dimensions of the motor, but it will be somewhere between 
20 and ioo per cent higher. 

Ordinarily, stationary motors are of the shunt-wound type, 
and such machines run naturally at practically constant ve- 
locity without regard to the size of the load. When a motor 
of this type is running with a full load, if the belt is thrown 
off, it will not increase its speed more than 3 or 4 per cent. 
Motors of this type are called shunt-wound because the current 
that passes through the field coils does not pass through the 
armature, but is shunted from the latter. 

These motors run at a constant speed without regard to the 




fig. 187. 



fig. 188. 



load they carry because the current that passes through the 
field coils remains constant no matter how much that through 
the armature may vary. Owing to the constant strength of 
current in the field coils, the strength of the magnetic field in 
which the armature revolves remains constant. The speed at 
which a motor armature will revolve in a constant magnetic 
field is dependent upon the voltage of the current. This voltage 
is counteracted by the back pressure developed by the motor 
armature and also by the voltage required to overcome the 
armature resistance. In shunt-wound motors, the armature re- 



282 



CHANGING THE SPEED OF MOTORS. 



sistance is so low that the voltage required to overcome it is 
only 2 or 3 per cent of the total; so that the back pressure of 
the armature, or counterelectromotive force has to attain nearly 
the same magnitude whatever the load on the motor may be. 

Since the constant current through the field develops a con- 
stant magnetic strength, and since in a magnetic field of con- 
stant strength the armature back pressure is constant at a given 
speed, it follows that the only variation there can be in the 
speed of the armature of a shunt-wound motor is that due to 
the slight difference in the amount of voltage balanced by the 




fig. 189. 



armature resistance with weak and strong currents; and this 
varies from nearly nothing, at light load, to 2 or 3 per cent 
of the total line voltage at full load. 

From the foregoing explanation it will be seen that there 
are two ways in which the speed of shunt-wound motors may 
be varied, one by changing the strength of the current flowing 
through the field coils, and the other by placing in the armature 
circuit a resistance that will absorb some of the line voltage, 
thus leaving less for the armature back pressure to balance. 
By means of the last named expedient the speed of the motor 



CHANGING THE SPEED OF MOTORS. 



283 



can be reduced, since the armature will have to develop a lower 
back pressure. By means of the first-named method the speed 
of the motor will be increased, because, if resistance is intro- 
duced in the field circuit, and the current is thereby reduced, 
the magnetic force will be reduced, and as a result the arma- 
ture will have to revolve faster to develop the required back 
pressure. 




FIG. I90. 



Fig. 189 illustrates the way in which a shunt-wound motor 
is connected so as to increase its speed by inserting resistance 
in the field circuit. The resistance in the field circuit is shown 
at R, and by making it in the form of a controller — so that more 
or less of it may be put in service — the increase in speed can 
be graduated. This arrangement can also be used to make the 
motor variable in speed and controllable by hand, but the varia- 
tion in velocity will be from the normal speed to higher speed. 



284 CHANGING THE SPEED OF MOTORS. 

The resistance R may be made of small wire because it has 
to carry only the field current, which is generally a small frac- 
tion of the armature current, from 1J/2 per cent in large motors 
to 5 or 6 per cent in small ones. The resistance of R y however, 
will have to be large to make any great change in the speed. The 
ordinary field regulators used with generators can be used for 
this purpose, a regulator for a one-hundred light machine being 
about the proper size for a 10-horsepower motor of the same 
voltage. If one regulator does not give all the change in speed 
desired, use two connected in series. 

Fig. 190 shows the way in which a shunt-wound motor is 
connected for varying the speed by inserting resistance in the 
armature circuit. By this means the motor can be made to run 
slower than the normal velocity, and it can be used as a variable 
speed motor controlled by hand regulation. In this diagram R 
is the ordinary motor starter, and R' is the resistance cut into 
the armature circuit. This resistance R' may be an ordinary 
motor controller, such as is used with series-wound motors of 
the same size and voltage. If it is desired with this arrangement 
simply to reduce the speed of the motor, the switch S' is turned 
until the proper velocity is obtained. If it is desired to vary the 
speed, the switch S' is moved as often as a change in speed is 
desired. The starter R cannot be used in place of the controller 
R' simply because it is not of sufficient capacity to carry the cur- 
rent continuously. 



MOTOR STARTERS AND CONTROLLERS. 285 



CHAPTER XXXVIII. 
Motor Starters and Controllers. 

IN CHAPTER XXXIV the general principles upon which 
motor starters are constructed were fully explained. In 
this chapter, and in others to follow it, it is proposed to 
illustrate and explain a number of the most commonly used 
motor starters and speed controllers. 

Motor starters are used for the purpose of starting a motor, 
only. Motor controllers are intended to regulate the speed at 
which the motor runs after it is in operation. Both devices are 
made so as to be used in connection with motors intended to run 
in one direction or in both directions. The construction of a 
motor starter is such that it controls the speed at which the 
motor runs in the act of starting. The construction of motor 
controllers is such that they can control the speed of the motor 
all the time. Thus it will be seen that both devices really act in 
the same manner, and, with the exception of a few differences 
in the details of construction, they are substantially the same. 
Controllers, however, are more massive in construction, and are 
able to carry large currents for long periods of time. 

It is undesirable to use a motor starter that will perform 
only the function of starting a motor. If a motor is running 
and the current in the line fails for any reason, the machine will 
come to a stop ; then if the current comes on again, it will catch 
the motor with the armature connected in the circuit without any 
resistance. As a result there will be sent through it a current 
strong enough to burn it out in a few seconds. Because of this 
fact it is necessary to provide a protective device that will open 
the motor circuit, if the current fails for any reason. Motor 
starters with this safety feature added to them are called "rio- 
vcltagc" starters. 

When a motor is running, if the load upon it is increased, 
the current will also increase, so as to give the machine the ad- 
ditional power required to carry the extra load: If the load is 
increased sufficiently, the current passing through the armature 



286 



MOTOR STARTERS AND CONTROLLERS. 



will be strong enough to burn it out. It is desirable, therefore, 
to have a protective device that will disconnect the motor be- 
fore the current can become so strong as to injure the armature. 
Motor starters are made with such a device, and when they have 
this in connection with the no-voltage attachment, they are 
called "no-voltage and overload" starters. A type of no-voltage 
starters is shown in Fig. 191. It is manufactured by the Cutler- 
Hammer Manufacturing Co., Milwaukee, Wis. The course of 




fig. 191. 



the current in passing through such a starter is shown in Fig. 
192. 

In this diagram the lines P N represent the line wires, and 
M is a double-pole main switch by means of which the motor 
circuit may be disconnected from the main line. At f, f safety 
fuses. are provided which melt and open the circuit if the current 
becomes too strong. 

It will be seen that the wire a connects the left side of the 
switch M with the lower brush of the motor, and also, through 
the wire g r , with one end of the field coils. Wire b runs from the 
right side of M to the binding post G at the bottom of the motor 



MOTOR STARTERS AND CONTROLLERS. 



287 



starter. This is connected with the stud D, around which the 
switch arm A swings. If A is moved to the right, as soon as it 
makes contact with the first of the contacts E, the main current 
passes through the resistance in the starter, which is indicated 




fig. 192. 



by the loops R, and reaches wire d, through which it passes to 
the upper brush of the motor; thence through the motor arma- 
ture to wire a, and through the latter back to the main line. 
Current for the field coils of the motor branches off from the 



288 MOTOR STARTERS AND CONTROLLERS. 

first E contact at the left, through wire e to a magnet coil C; 
thence to wire g, and through the motor field coils to wire g , 
which connects to the wire a. 

Current passing through the magnet C energizes it so that 
when the switch arm A is moved as far as it will go to the right, 
and the piece B rests against the poles of C, the attraction of 
the latter will hold A, the piece B being made of iron. As is 
shown in the diagram, there is a spring around the stud D, 
which spring acts to swing A around to the stop position; but, 
as long as a current passes through C, the attraction of the lat- 
ter is more than the spring can overcome. If the current from 
the line fails, magnet C becomes de-energized, and the spring is 
free to swing A around to the stop position. If the line current 
is then re-established, the motor is not caught connected in the 
circuit without resistance in series with the armature. A stop 
is provided at F, so as to prevent the spring around D from 
swinging A too far. 

In the second diagram, Fig. 193, the connections within the 
motor starter are substantially the same as in Fig. 192, and the 
releasing magnet C is actuated in the same manner. An addi- 
tional connection is made between wire e and the iron core of 
magnet C, through wire e" , so that when B rests against the 
poles of C the current may pass through the coil of the latter 
directly from A without having to go to the right-hand contact E 
and thence through the resistances R to wire e. The effect of 
this arrangement is to slightly increase the strength of the mag- 
net, and to provide an additional path for the current, so that if, 
for any reason, the circuit through the resistances R should be 
broken, there would still be another path through e" and the core 
of C. There is a spring around the stud I which acts to swing 
the switch lever A around to the open position whenever the cur- 
rent through the motor fails, precisely the same as in Fig. 192, 
but it is not shown in the diagram. 

Fig. 193 is the type made by the Cutler-Hammer Co. for 
motors, using currents not exceeding 50 amperes. For larger 
motors, this company provides the starter, which is diagram- 
matically shown in Fig. 194. In this design, the circuit connec- 
tions are the same as in the two preceding diagrams, Figs. 192 
and 193, with the exception that when the lever A is raised to 



MOTOR STARTERS AND CONTROLLERS. 



289 



the vertical position and all the starting resistance R is cut out 
of the armature circuit, a spring connecting piece D is forced 
into contact with the blocks E E, thus providing another, and 




fig. 193. 

more direct, path for the main current between wires a and d. 
The iron block B on lever A, is located at the extreme end, and 



290 



MOTOR STARTERS AND CONTROLLERS. 



magnet C is placed outside of the resistance contacts //, thus 
enabling a comparatively small magnet to force the connector D 
against EE with sufficient pressure to make a good contact. 




fig. 194. 



One advantage of this construction is that the contacts // need 
not be as large as when the blocks E E are not used, and, in 
addition, if, for any reason, the connections in the resistance R 



MOTOR STARTERS AND CONTROLLERS. 29I 

become broken or imperfect, the current can find a path through 
E E and the connector D. 

In these three diagrams it will be noticed that the connec- 
tions between the motor and the starter are such that the circuit 
through the field coils of the motor is never opened. As was 
fully explained in Chapter XXXVI, this arrangement is neces- 
sary to prevent serious sparking at the last contact to the left of 
the starter, when the motor is stopped, and also to avoid in- 
juring the insulation of the motor field coils. In Fig. 192, if the 
circuit is traced from the upper motor armature brush it will 
be found that it passes through wire d to the right-hand con- 
tact E) thence through the resistances R to wire e, through mag- 
net C to wire g, through the motor field coils to wire g , through 
wire a to the lower motor brush, and finally through the arma- 
ture to the upper brush, which is the starting point. Thus it will 
be seen that the motor field and armature are connected so as 
to form a loop, or closed circuit. In Fig. 193, if we start from 
the upper motor brush through wire d, we come back through 
wire g to the motor field through wires g r and b, to the lower 
brush, and through the motor armature to the starting point. 
This is also the connection of the armature and field shown in 
Fig. 194. 

Plain motor starters that are not provided with an automatic 
releasing magnet C are sometimes connected after the manner 
shown in Fig. 195, but this is not an arrangement to be recom- 
mended. In looking at this diagram it will be seen that, if the 
main switch B is closed, the circuit through the field coil D of 
the motor will be closed. If now the switch lever C of the motor 
starter is moved to the right, over the contacts E, the circuit 
through the armature A of the motor will be closed, and the 
motor will be set in motion. 

If when the motor is running we open the main switch B, 
the motor will be disconnected from the main line P N, and the 
circuit through the motor field will not be opened. As the line 
current is cut off, the motor will come to a stop, and then we 
can move switch lever C to the open position without doing any 
harm. If we should undertake to stop the motor by opening 
switch lever C instead of B, the result would be a considerable 
sparking at the contacts E, and if we then opened switch fi to 



292 



MOTOR STARTERS AND CONTROLLERS. 



disconnect the motor from the main line, the circuit through the 
field D of the motor would be opened, and if the machine were 
large, the probabilities are that the insulation would be dam- 
aged. 

The objection to arranging the circuits of the motor and 
starter in the manner shown in this diagram is that while with 
it the motor can be stopped without injury, there is danger of 
the switch levers not being manipulated in the proper order, 




fig. 195. 

either through ignorance or carelessness. Furthermore, if .the 
switch B is opened first, as it should be, there is danger of for- 
getting to open C when the motor comes to a stop. And in such 
an event there would be great liability that the switch B might 
be closed to start up the motor, without first returning C to the 
open position. 



NO-VOLTAGE AND OVERLOAD STARTERS. 



293 



CHAPTER XXXIX. 

No- Voltage and Overload Motor Starters. 

IN THE LAST chapter we described automatic no- voltage 
motor starters, which are also called underload automatic 
starters. Figs. 196 and 197 are now given to illustrate no- 
voltage and overload starters, the first being made by the Cutler- 




fig. 196. 



Hammer Co., and the second by the Ward Leonard Electric Co. 
Fig. 196 shows an arrangement which combines with the motor 
starter proper, a two-pole main switch, and two safety fuses. 
Fig. 197 might be arranged in the same way. 

For Fig. 196 the circuit connections are shown in the dia- 



294 



NO-VOLTAGE AND OVERLOAD STARTERS. 



gram, Fig. 198. As will be seen, the main switch M has its 
upper left-side terminal connected with the N line through wire 
e e' and the lower right-side terminal with the P line through 
wires gg', the safety fuses being located at f f. From the lower 
left-hand terminal of M, wire a runs to binding post G and the 
upper right-hand terminal of M is connected through wire b 
with the lower brush of the motor armature. From G through 
wire h the circuit runs to magnet D through the coil of which 
the entire current that actuates the motor is passed. From D 




•fig. 197. 



through wire c the circuit runs to switch A, and when this is 
moved over the contacts E the circuit continues through wires 
d, d' and d" to the top motor brush, thence through the motor 
armature to wire b and back to the main line. 

From the left-hand E contact a wire i is run to magnet C 
and then through wires j, f and k to the motor field coil, from 
which the circuit continues through k' to wire b. It will be seen 
that magnet C is connected in the same way as is the same mag- 
net in the no-voltage starter, and it acts in the same manner ; 
that is, it holds lever A in the extreme right-hand position 



NO-VOLTAGE AND OVERLOAD STARTERS. 



295 



against the tension of a spring wound around the stud upon 




fig. 198. 
which A swings. The magnet D is traversed by the whole cur- 



2g6 NO-VOLTAGE AND OVERLOAD STARTERS. 

rent, hence its strength increases as the whole current increases, 
and when the latter reaches the strength for which the magnet is 
adjusted, the armature F is lifted and its end connects the ter- 
minals n n'. In this way the magnet C is short-circuited and 
loses its strength, permitting the spring to swing A around to 
the open position, and stop the motor. From this description it 
will be seen that magnet C acts precisely the same as in the no- 
voltage starter, and that the office of magnet D is to shut off the 
current from C whenever the current passing through the motor 
is strong enough to lift the armature F. 

Automatic motor starters, whether of the type shown in the 
last chapter or like those here presented, hold the lever A firmly 
in the extreme right-hand position when the motor is running 
and the latter cannot be stopped by moving A back to the stop 
position, unless a considerable force is employed. To stop mo- 
tors provided with such starters, the main switch M is opened 
and then, as soon as the current through the motor dies out, the 
magnet C loses its strength and allows the force of the spring 
around the stud of A to swing the latter to the open position. 
With the overload starters, it is possible to stop by simply lifting 
the armature F so as to short-circuit magnet C. 

As it is a very easy matter to lift F, much easier than to 
open the main switch M, some men get in the way of resorting 
to this method of stopping the motor; but it is not advisable to 
follow the practice, because, when this is done, the circuit con- 
nection between lever A and the contacts E is broken while the 
full current is passing, and as a result there is considerable 
sparking at the contacts E, which in time gets them so rough as 
to prevent the switch from working freely. 

Fig. 199 shows the circuit connections, for the starter illus- 
trated in Fig. 197, all parts of which, except the overload magnet 
G, act in the same manner as in Fig.; 198. The action of the 
overload magnet, however, is quite different. The lever D is 
held in position by the catch F and a spring around stud / acts 
to swing D upward. The end of D rests upon a contact with 
which wire c is connected, so that, with the parts in the position 
shown in the diagram, the current from A passes through D 
to wire c and thus to line wire N. The magnet G is of the sole- 
noid type and exerts a force to lift the plunger s. When the 



NO-VOLTAGE AND OVERLOAD STARTERS. 



297 



main current becomes strong enough, magnet G lifts plunger s 
and the latter, striking a blow against the end of F, throws the 
catch at its upper end out of engagement with the lever D, when 




fig. 199. 



this lever, actuated by the spring around stud /, flies upward and 
breaks the contact between wire c and switch A, thus opening 
the circuit throusrh the motor armature. 



2g8 NO-VOLTAGE AND OVERLOAD STARTERS. 

As will be noticed in Fig. 197, the plunger ^ is guided by a 
frame attached to the lower side of magnet G. This plunger 
is held in its normal position by means of a set screw which is 
seen projecting below the frame, and by adjusting this screw, 
the device can be set so as to cause the plunger ^ to lift with 
different strengths of current. There is a scale marked in am- 
peres on the front of the frame that guides s, and attached to the 
latter there is a pointer that moves over this scale, so that, by the 
movement of the adjusting screw, the magnet can be set to act 
at any desired number of amperes. With this starter, the motor 
can be stopped by lifting F so as to release D, but, as stated in 
connection with Fig. 198, the practice is a bad one, and should 
not be followed. 

It will be noticed that in Fig. 198 fuses are shown at ff, 
while in Fig. 199 there are none. It may be asked why they are 
used in one case and not in the other; and, since the overload 
magnet acts to open the circuit when the current becomes too 
strong, why are fuses used at all ? In answer to these questions 
it may be said that this type of starter can be used without fuses, 
if desired, as the overload magnet is generally sufficient protec- 
tion. Fuses can be used with Fig. 199 just as well as with Fig. 
198. 

All things considered, it is advisable to use the fuses, be- 
cause they act in a manner somewhat different from that of the 
overload magnet, and hence afford additional protection. The 
overload magnet will respond to a very sudden increase in cur- 
rent, even if it lasts for only a short time, and on that account 
gives complete protection for the motor against sudden rushes of 
current. The safety fuse will not respond to a sudden increase in 
current because it requires some time to heat the fuse wire up 
to the melting point, but sufficient increase in current, if con- 
tinued, will melt the fuse. 

Fig. 200 shows a type of motor starter made by the Cutler- 
Hammer Co. for use in connection with very large motors, 100 
horsepower or more. The complete apparatus is shown as filling 
one whole panel of a switchboard, the diagram of wiring con- 
nections for which is shown in Fig. 201. At the top of the panel 
is located a circuit breaker which acts in the same manner as 
the overload magnet in the starters already explained. The 



NO-VOLTAGE AND OVERLOAD STARTERS. 299 

magnet of this circuit breaker is shown at F, and is arranged so 



B^ 




FIG. 200. 

as not to be traversed by the whole of the main current, as this 



300 NO-VOLTAGE AND OVERLOAD STARTERS. 

would require very large wire. The current for this magnet is 
shunted from the ends of the bent bar I, which is so made as to 
offer proper resistance to force the required current through the 
magnet. The lever of the circuit breaker connects the contacts 
L L, and the circles gg represent magnets used to extinguish the 
spark produced when the contact across L L is broken. The 
switch at the lower end of the panel is the main-line switch, 
which, in the closed position connects the contacts 5 S and S' S' , 
the pairs on the right and left sides, respectively, being connected 
with each other. 

Along the center of the panel is a row of switches for the 
purpose of cutting out the resistance in the armature circuit; 
that is, they take the place of switch lever A in the other starters. 
These switches are made to interlock each other so that No. 2 
cannot be moved until No. 1 is closed, and so on for all the 
others. The small magnet n' is for the purpose of holding these 
switches in position and of releasing them all when the main 
switch is opened. At G a pilot lamp is placed to indicate whether 
there is a current in the circuit before the switch is closed, and 
also for the purpose of lighting up the panel when desired. At H 
is placed a switch to close the lamp circuit when the main switch 
is open. This switch is opened before the main switch is closed. 
When the main switch is open, the contacts p and q are con- 
nected with the S' and S contacts directry above them. 

When the main switch M is closed, the current from main- 
line wire P in wire b passes through g' and the blow-out magnets 
g to wire h and to contact plate C, thence by wire K to magnet n 
and to wire i, through lamp G to i and contact p of main motor- 
starter switch. If switch H is closed, the current will pass to q 
and thence to the lower contact .S above it, to wire a which runs 
to the opposite side N of the main line. If the circuit breaker F 
is now closed, the main current will flow through I to contacts 
L L, to wire c and to the upper S' contact 01 me main switch. 
If this switch is also closed, the current from the upper S' will 
pass to the lower S' and thus through the motor armature to 
wire e' and to contact plate D. 

If the first or left-hand switch of the center row is now 
closed, the main current will pass from D through the 
starting resistances R to contact plate C , through the switch lever 



NO-VOLTAGE AND OVERLOAD STARTERS. 



301 



to contact B, thence through wire d to the upper contact .5", 

P 







miMFi\ 




FIG. 201. 
through the main switch lever to the lower 6" and to wire a, thus 



302 NO- VOLT AGE AND OVERLOAD STARTERS. 

returning to the main line. The field current will branch from 
wire / through wire k', and from the field will go to contact 
plate C through wire k. The plate A is connected with C by 
wire e so that, as the switch levers are successively pressed into 
position, the contacts E are connected with A, one after the 
other, thus cutting out in succession the several sections R of 
the starting resistance. As switch H is open when the motor is 
running, the current through magnet n will break as soon as 
the main switch breaks the connection between the contacts 5" 5 
and S f S\ 



MOTOR CONTROLLERS. 



303 



CHAPTER XL. 

Motor Controllers. 

FIG. 202 shows a controller that is arranged to vary the speed 
of the motor by cutting resistance into the armature cir- 
cuit and also into the field circuit. By cutting resistance 
into the armature circuit the speed is reduced, and by cutting it 
into the field circuit the speed is increased. Fig. 203 shows a 




fig. 202. 



controller that is arranged to vary the speed of the motor in 
the same way as Fig. 202, and in addition is provided with 
means for stopping the motor quickly. This quick-stopping ad- 
dition is very desirable in connection with motors used to operate 
printing presses and also for many kinds of motor-driven ma- 
chine tools. Both illustrations are of controllers made by the 
Cutler-Hammer Manufacturing Co. 



304 



MOTOR CONTROLLERS. 



Circuit connections for Fig. 202 are shown in the diagram, 
Fig. 204. The connections are substantially the same as for a 
motor starter, the wire a running from one side of the main 
switch M to one of the motor brushes, and the other side of 
the switch being connected with the switch lever A through 
wires b and c. The resistances represented by the loops R, which 
connect with the segment G, are in the circuit of the armature 
of the motor, and the resistances represented by the small loops 



w" ' 7 




EIG. 203. 



r, which are connected with the segment E, are connected in the 
motor field circuit. 

Magnet C acts the same as in the motor starters, to open 
the circuit through the motor armature whenever the current 
dies out in the main circuit. The segment B which is attached 
to the lower end of lever A is provided with teeth, as indicated 
at the upper portion, these teeth covering the whole segment. 
The armature D of magnet C carries a spring catch at the 
outer end, which engages with the teeth on the segment B, when 
D is drawn down into the position shown in the diagram, by 
the attraction of C. 



MOTOR CONTROLLERS. 



305 



The resistances R are made of sufficient size to carry the 
whole current that passes through the motor armature for any 
length of time without getting too hot. On this account, if it 
is desired to run the motor at a low velocity, the lever A is 
moved over the contacts connected with the resistances R until 




fig. 204. 



the proper speed is. obtained, and is left in that position. The 
catch on the end of D engages with segment B in this position 
and prevents A from being thrown back to the stop position by 



306 MOTOR CONTROLLERS. 

the force of the spring that is placed around the stud upon which 
the lever swings. This spring is not shown in the diagram, but 
is mounted in the same way as on the motor starters. 

If it is desired to run the motor at its normal velocity, the 
lever A is moved around until it comes in contact with segment 
G and thus cuts out all the resistance in the circuit of the motor 
armature. If a still higher velocity is required, the lever is 
carried further ahead, so as to pass off the segment E and rest 
on one of the small contacts connected with the resistances r. 
In this way resistance is cut into the field circuit of the motor, 
and, owing to the reduced strength of the field, the speed of 
rotation of the armature is increased. 

When lever A is moved to the extreme right-hand position, 
the motor will run at the highest velocity, and when A is on the 
first contact at the left, the speed will be the lowest. In either 
one of these two positions, or in any intermediate position, lever 
A is firmly held by the catch attached to D so long as cur- 
rent passes through the motor. If, however, the line current is 
interrupted from any cause, the magnet C loses its strength, D 
can no longer hold the catch against B, lever A swings back to 
the stop position, and there is no danger of injuring the motor 
armature if the line current is re-established. 

For the controller shown in Fig. 203 the wiring connections 
are given in the diagram, Fig. 205. This controller, as already 
stated, is the same as Fig. 202, with the addition of means for 
stopping the motor quickly, which is effected by converting the 
motor into a generator, so that the momentum of the armature 
is absorbed in developing a current; in other words, the power 
given out by the armature as a generator acts as a brake to 
arrest the motion. 

For this controller the operation is as follows : If the lever 
A, Fig. 205, is moved to the right from the position in which it 
is shown, so as to cover segment F , the main current which 
comes from wire c to F will pass through A to the contact E, 
upon which A may be resting; then through the resistances r 
to segment G and thus through wires h and K to the motor 
armature, and from the latter through wires b' and b to the op- 
posite side of the main line. If lever A rests on the first E con- 
tact at the left, the motor speed will be the lowest, while if it 



MOTOR CONTROLLERS. 



307 



rests at the extreme right side of G, the speed will be the high- 
est, precisely as in the case of Fig. 204. 

When A is in the position shown, the circuit through the 




crf^ > Cf ^r! 




FIG. 205. 

armature of the motor is broken, as the current from F cannot 
pass in any way to G. If A is moved to the left so as to cover 
contact F', then this contact will be connected through the segment 



308 MOTOR CONTROLLERS. 

H and the resistances r with segment G, and thus the circuit of 
the motor armature will be closed and any current generated in 
the latter will be forced through the resistances r. It will be 
noticed that when lever A is in the left-hand position, resting 
on F r , the circuit through the field of the motor is closed, as 
segment F is connected by wire i with the right-hand contact of 
the row connected with the resistances r, so that the line cur- 
rent can pass to wire e and thus through magnet C to wire e" , 
and through the motor field to wire g and the opposite side of 
the main line. 

If the contacts of the controller are arranged precisely as 
shown in Fig. 205, the current generated in the motor armature 
when A is moved to the left position will be such as the mag- 
nitude of the total resistance r will permit it to be. Under these 
conditions the motor may stop more quickly than desired, or not 
quickly enough. If the stop is not quick enough, the segment H 
may be made shorter and a separate contact provided to the left 
of it, this contact to be connected with any point of the resist- 
ance r which may be found necessary to effect a stop in the 
proper time. Or, if the stop with all the resistance r in the 
circuit is too rapid, an additional resistance can be cut into the 
circuit. The contacts and the resistances can be arranged so as 
to adjust the rapidity of the stop to suit any particular case. 

Small circles .? ^ are stops to prevent swinging lever A too 
far in either direction. Whenever it is desired to make a slow 
stop, lever A is returned to the position in which it is shown, 
but if a quick stop is desired, it is carried around to the left 
until it strikes the stop s. The spring that swings the lever back 
to the stop position is mounted upon the central stud around 
which the lever swings. 

Whenever the motor is stopped for any length of time, the 
main switch M is opened so as to break the circuit through the 
field coils. The fuses / f protect the motor against a strong 
current, and magnet C protects it against sudden stoppage of 
current in the main line, so that the machine is as well guarded 
as if connected with an overload and no-voltasre motor starter. 



REVERSING MOTOR CONTROLLERS. 



309 



CHAPTER XLI. 
Reversing Motor Controllers. 

IN MANY cases it is desired to be able to run a motor in 
either direction, and for that purpose a reversing controller 
is required. Fig. 206 is a controller which is arranged to 
run the motor at full speed in either direction, but, if provided 
with resistance of sufficient capacity, may be used to obtain 




FIG. 206. 

different speeds by cutting resistance into the armature circuit. 
Fig. 207 shows another form of reversing controller, made by 
the same firm, the Cutler-Hammer Mfg. Co., with which a num- 
ber of different speeds may be obtained in one direction, and 
two speeds when running in the opposite direction. Controllers 
of this type are used in cases where it is desired to run at 



310 REVERSING MOTOR CONTROLLERS. 

several speeds in the forward direction, which is the direction 
in which the motor is run most of the time, and at only one or 
two speeds when the motor is reversed. 

Circuit connections for Fig. 206 are shown in Fig. 208. The 
current from main line P passes through wires b and b' to the 
upper binding post 7/, and thence through wires d' and d to the 
left-hand contact E '. Through wire g the current reaches the 
motor field and thence passes through wires e" and c' to mag- 
net D, through wire e to wire c" , to contact u, to wire c , through 
u to wire c and to wire a, which connects with the main line N. 




FIG. 207. 

The magnet D acts to hold lever A in any position in which it 
may be placed, precisely the same as the magnet C in the con- 
trollers explained in the last chapter. The lever A is divided 
into two parts, A and B, by an insulating section marked t. 

If the main switch M is closed and the lever A is in the ver- 
tical position, the circuit through the motor field will be closed, 
as will be seen by following wire g; but the armature circuit will 
not be closed, as there is no connection between the contacts E E' 
and F F\ The contacts E E' are connected with each other by 



REVERSING MOTOR CONTROLLERS. 



311 



the wires i i, and contacts F F' are connected with G G' by means 
of the wires h K . 

If lever A is moved to the right, the section above t will 




FIG. 208. 



connect F with the contacts E, and then the current from the E' 
contact at the left will pass through wire i to the E contact at 
the extreme right and through the resistance loops R until it 



312 REVERSING MOTOR CONTROLLERS. 

reaches lever A. Through this lever the current will pass to F, 
to the small contact u, to binding post v and to the lower motor 
brush. Returning from the upper motor brush, the current will 
reach G and, through the lower section B of the lever A, will 
reach the stud around which the lever swings and with which 
wire c" is connected. From this point through wires c and c 
the lower binding post v' is reached, and thus wire a, which is 
connected with the N side of the main line. 

In this case it will be observed that the current reaches the 
motor armature through the lower brush. Now, if the operating 
lever is moved to the left, it will be found, by tracing the circuit 
through the contacts E and E' , the connecting wires i and resist- 
ance loops R, that the current from the extreme left-hand E' 
contact will reach lever A wherever it may be resting on the E' 
contacts, will pass to F' , and thence through wire h will reach 
the upper motor brush, and will return from the lower brush 
through wire li to contact G' , and thence through section B of 
lever A to wire c" and back to the A r side of the main line 
through wires c , c and a. 

Thus it will be seen that if, when the operating lever is 
moved toward the right, the armature rotates clockwise, when 
the lever is moved to the left, the armature will rotate counter- 
clockwise ; for in the first case the current will enter the arma- 
ture through the lower brush, while in the second it will enter 
through the upper brush. 

The piece / is not connected with the circuit, and is simply 
provided to form an even path for the lever to move over. The 
small contacts marked u and u', four placed in a row on each 
side of the main contacts, are for the purpose of making a more 
perfect connection when the operating lever is in the extreme side 
position. The spring connectors marked s s press against these 
contacts when in the side position. The actual form of these 
connectors, and of the u and u contacts, is well shown in Fig. 
206. 

As in the two controllers described in the last chapter, the 
segment C is provided with teeth on its periphery, and the spring 
catch attached to the armature H of magnet D engages with 
these teeth to hold the operating lever in any position. As in 



REVERSING MOTOR CONTROLLERS. 3 13 

this type of switch the lever swings in both directions, springs 
are placed around the stud that act to bring it to the central 
position when moved either one way or the other. The arrange- 
ment of these springs can be seen in Fig. 206. 

For the controller shown in Fig. 207 the circuit connections 
are given in the diagram, Fig. 209. This controller, as already 
explained, is arranged so as to give several speeds in one direc- 
tion, but only two in the other direction. The motor shown in 
this diagram is of the compound type, the shunt field coils being 
marked S S and the series coils m m. The magnet D acts in 
the same way as in Fig. 208. The shunt field current branches 
from wire b through wire g, and passes from the field coils to 
wire e" , up to the binding post v } to wire e , thence through mag- 
net D to wire c and segment L. The main current passes 
through wire b to the series field coils m m, through wire b' 
to binding post 1/ and thence to contact /. The lower motor 
brush is connected through wires d' and d with contact G', and 
the upper brush is connected through wires K and h with con- 
tact G. The N side of the main line is connected with the stud 
around which the operating lever swings, through wires a and c. 

If the lever is moved to the right far enough to cover the 
first E contact, the current from / will pass through all the r 
resistance to A, thence to F, through wire % to G', to wires d and 
d', and to the lower side of the motor armature. From the upper 
armature brush the current will return through wires ti and h to 
G, and through section B of the operating lever to wire c, to wire 
a, and to the opposite side N of the main line. 

As the position of A is advanced toward the right, section 
after section of the resistance r is cut out and the motor speed 
is correspondingly increased. When A reaches contact / the 
normal motor speed is obtained, and if it is advanced still farther 
along I, the sections of the resistance r are cut into the circuit 
of the shunt field coils 6" S and the speed of the motor is further 
increased. Thus it will be seen that as many changes in velocity 
may be obtained as there are E contacts, plus the contacts con- 
nected with the r[ resistance in the field circuit. 

If the operating lever is moved to the left, the motor is re- 
versed, as the F' contact is connected with G, so that the current 
from contact I , after passing through all the r resistances to one 



314 



REVERSING MOTOR CONTROLLERS. 



of the wires n and the corresponding E' contact, will, through 

A, reach F'. Then, through wire i, the current will pass to G, 

P 



JV 



F 



c^^iL^o 




FIG. 209. 

and through wires h and W to the upper motor armature brush, 
which is the reverse of the direction when the operating lever 



REVERSING; MOTOR CONTROLLERS. 3 15 

is moved to the right. If the lever is moved far enough to cover 
the first E' contact, all the r resistances will be left in the cir- 
cuit of the motor armature, but if it is moved to the second E' 
contact, one section of this resistance will be cut out. As may 
be readily seen, the second E' contact may be connected with 
other E contacts, so as to cut out two or more sections of the r 
resistance, and thus give a greater difference between the two 
speeds obtained in the reverse motion. The upper end A' of the 
operating lever is connected with the lower section B, so as to 
keep the shunt field circuit closed when the lever is moved to the 
stop position — i. e., the position in which it is drawn. 



3i6 



MOTOR CONTROLLERS FOR PRINTING PRESSES. 



CHAPTER XLII. 
Motor Controllers for Printing Presses. 

IN MANY cases it is desired to have the controller so ar- 
ranged that the motor may be stopped quickly from several 
different positions. For such service it is evident that the 
simple arrangements heretofore shown cannot be used, because, 



1 



with them, the motor can be controlled from only one position, 
and that is the place where the controller is located. In the 
operation of a printing press, the work being done has to be 



MOTOR CONTROLLERS FOR PRINTING PRESSES. 317 

observed from several different positions, and the observers at 
any of these points should be able to stop the machine instantly 
if anything goes wrong. 

The controller shown in Fig. 210, made by the Cutler-Ham- 
mer Co. for printing press service, is arranged so that, while the 
motor is started by the movement of the controller lever, it can 
be stopped by simply pressing a push button located at any de- 
sired point, and there may be any number of these buttons located 
wherever they may be required. To accomplish this result, the 
main switch of the controller is arranged so as to be actuated 
by a magnet. When a current is passed through the coil of 
this magnet, the switch is closed and the motor is properly con- 
nected with the main circuit. 

When the current through this magnet is interrupted, the 
main switch is opened and thus the motor circuit is broken. 
The circuit through the coil of the switch-operating magnet is 
extended so as to include all the push buttons by means of 
which the motor is to be stopped. These buttons are connected 
so as to keep the circuit normally closed, but when any one of 
them is depressed the circuit through the switch magnet is 
opened, and the switch then opens the motor circuit. 

In Fig. 210 the magnetic main switch is seen at the lower 
end of the panel, a little to the right of the center line. The 
small magnetic switch to the left of this is an overload device, to 
open the circuit in case the current becomes too strong. The 
two incandescent lamps on top of the controller are used to re- 
duce the strength of the current that passes through the mag- 
net of the main switch after the latter has been lifted into the 
closed position. The magnet of this switch is of the solenoid 
type, and in these magnets the current required to lift the plunger 
when it is in its lowest position is much greater than that neces- 
sary to hold the plunger up after it has been raised to its high- 
est position. By cutting the two lamps into the magnet circuit 
after the switch has been closed, the current strength is greatly 
reduced; thus energy is saved and, in addition, the magnet coil 
is prevented from becoming overheated. 

Fig. 211 shows the general arrangement of this, controller. 
The magnet of the main switch is shown at B ; when current 
passes through this magnet, the plunger /is raised and the con- 



3i8 



MOTOR CONTROLLERS FOR PRINTING PRESSES. 



nector attached to its lower end joins the contacts vv, thus clos- 
ing the circuit through the motor. When the current through B 




. FIG. 211. 



is shut off, the plunger / drops, thus opening the circuit through 
the motor armature. The small switch n is in the position indi- 



MOTOR CONTROLLERS FOR PRINTING PRESSES. 319 

cated in dotted lines, when the circuit through B is open, and 
rests upon the contact t. The catch k holds n in this position 
and a spring acts to pull it into the position in which it is shown. 

In the circuit i the switches s' s' represent the push buttons 
that are located at the several points from which it is desired to 
stop the motor. If all these s' switches are closed, the closing 
of switch s, located on the controller panel, as shown in Figs. 
210 and 211, will close the circuit through B, and thus connect vv 
and establish the circuit through the motor. The upward move- 
ment of j will cause its upper end to strike k and release switch 
n, thus breaking the connection with t. When n rests on t, 
the wire h is in direct connection with h" through switch 
n, but when n is released and swings to the position in which 
it is drawn, the wire h is disconnected from h" , and the current 
in the latter must pass through the two lamps / / to reach wires 
h! and h. From this it will be seen that until the upper end of / 
strikes k the lamps / / are short circuited by the switch n, but as 
soon as this switch is released and swings away from t the lamps 
are cut into the circuit, and thus the current passing through B 
is greatly reduced, but not until / has been raised so as to con- 
nect the two contacts vv. 

Whether / is down or up, the circuit through B is closed, 
providing all the switches / and s are closed, for, as will be seen, 
if we follow wire b to lever A, we shall reach segment E, with 
which wire h is connected, and if n rests on t, the current will 
pass to h" and through B to wire i, through switch s to wire i' 
and to contact p. From this contact, through spring o', the cur- 
rent will pass to wire i", thence through magnet C to wire c, 
and finally to wire a, which is connected with the opposite side of 
the main circuit. 

When the main controller lever A is in the stop position, it 
presses switch n over onto contact t, thus cutting out the two 
lamps / /. If A is not in the stop position, switch n will not be 
pressed over onto t, and if such is the case, the current passing 
through B will not be strong enough to lift the plunger /. The 
result of this is that, if the current in the line should fail while 
the motor is running, and lever A is resting on one of the con- 
tacts D, the circuit will be opened through the motor on account 
of the current through B dying out, and as B cannot lift / with 



320 



MOTOR CONTROLLERS FOR PRINTING PRESSES. 



the two lamps / / in circuit, the motor circuit cannot be closed 
again until A has been returned to the stop position and has 
forced n over the contact t. In this way the motor is protected 
from the danger of being started with all or a large portion of 
the armature resistance r cut out of the circuit. 

Magnet C is for the purpose of protecting the motor against 
an excessive current due to an overload. When the main cur- 
rent, all of which passes through C, becomes strong enough, the 




fl\f7\ 





FIG. 212. 



armature o is lifted and thus breaks the connection between p 
and the spring o'. As will be seen, this break opens the circuit 
t, in which the magnet B is included, and causes the main switch 
to be opened by the dropping of plunger ;. The two coils L L 
are magnetic blowouts and are provided to blow out the sparks 
formed between the contacts v v and the connector carried by /, 
when the switch is opened. 

Fig. 212 shows a form of magnetic controller used to operate 



MOTOR CONTROLLERS FOR PRINTING PRESSES. 



321 



motors that are stopped and started automatically, or are actuated 
from a distant point, or in which it is desired to simplify the 
operation of starting. The diagram, Fig. 213, shows the way in 




\TIM 




FIG. 213. 

which the starter is connected when used for the purpose of 
simplifying the operation of starting. With this arrangement 
all the attendant has to do is to close the main switch M and the 
starter does the rest. As soon as M is closed, the circuit through 



322 



MOTOR CONTROLLERS FOR PRINTING PRESSES. 



the motor is closed; through wires k' and k the current passes 
to magnet C, through wire g" and switch i to the wire g', and 
thus to wires c and a and the opposite side of the main line. 




fig. 214. 

As soon as magnet C is energized, it draws up the plunger 
B and thus swings lever A over the contacts D and cuts out the 



MOTOR CONTROLLERS FOR PRINTING PRESSES. 323 

resistance in the circuit of the motor armature. The dashpot 
shown in Fig. 212 opposes the pull of magnet C and thus 
regulates the speed at which A is moved over the contacts. 
When A reaches the top position, it strikes the small switch i 
and opens the circuit with contact s, thus cutting in the two 
lamps indicated at E, or any other suitable resistance, so as to 
reduce the current passing through C. 

Fig. 214 shows how this controller is arranged to be actuated 
from a distance. In this case a magnetic main switch D is pro- 
vided, the circuit through which is opened and closed by a small 
switch p located at any point desired. When p is closed, the 
current passes through D to wire n, and to wire n, to wire i", 
and thence to wire i', which connects with a contact upon which 
lever A rests. Through A the current passes to wire a and to 
the opposite side of the main line. 

As soon as the current passes through D, it lifts its plunger, 
and thus the connector G joins the contacts vv and closes the cir- 
cuit through the motor. The operation of magnet C will now 
be the same as in Fig. 213. As soon as A moves upward, it 
passes off the contact with which wire % is connected, and then 
the current passing through D has to flow through the resist- 
ances at E to reach the opposite side of the line, and in this way 
the current through the main magnet D is cut down immediately 
after the connector G has been raised into position. As in Fig. 
213, when A reaches the top position it opens the switch so as 
to cut the resistance F into the circuit of C. 

This type of motor starter is used to operate automatically 
pumps that deliver water into a tank, where it is desired that the 
motor be stopped when the water reaches a certain level in 
the tank, or when the pressure reaches a certain point. In the 
first case, the switch p is actuated by a float in the tank, and in 
the second case it is actuated by a pressure regulator. 



324 MOTOR STARTERS WITH ELECTROMAGNETIC SWITCHES. 



CHAPTER XLIII. 
Motor Starters with Electromagnetic Switches. 

MOTOR starters of large size are made not only in the 
forms shown in previous chapters, but also with separate 
switches that are magnetically operated for making the 
various changes in the circuit connections. Starters and control- 
lers of this type are more elaborate and expensive than the de- 
signs in which the various circuit combinations are effected by 
the movement of a single switch lever that swings over a row 
of contacts, but to offset this increased cost, the separate switch 
construction gives greater wearing capacity and offers less liabil- 
ity to developing short circuits. 

In a starter such as is shown in Fig. 211, it can be easily 
seen that, if the. current handled amounts to several hundred am- 
peres, there is danger of seriously damaging the contacts D if 
the lever A fails to make a good connection with them as it 
swings upward ; and in descending the sparking will be quite 
severe unless there are a large number of contacts, so as to 
divide the resistance r into so many sections that the voltage re- 
quired to drive the current through each one is very small. 

When a switch of this type is new, if it is properly con- 
structed, there is no difficulty in obtaining perfect contact for all 
positions of the lever A, but the wear upon the rubbing surfaces 
will not be uniform and, as a result, in the course of time some of 
the D contacts will be lower than the others. This unevenness 
will cause the sparking to increase when the lever A swings 
downward, and the increase in sparking will result in more rapid 
wear, thus producing still greater irregularity in the surface. Un- 
less this inequality in wear is remedied by truing up the surface 
of the contacts, a time will come when the sparking at some 
point will be great enough to burn them out. 

Owing to these facts, the independent switches which are 
capable of withstanding harder usage are considered by many 
to be preferable, notwithstanding their greater cost. There 
are many designs of independent switch motor controllers and 
starters, some being comparatively simple and others very elab- 



MOTOR STARTERS WITH ELECTROMAGNETIC SWITCHES. 325 

orate. One of several designs made by the Cutler-Hammer 
Co. is shown in Fig. 215, and the diagram of the circuit con- 
nections is given in Fig. 216. The switch in the lower left-hand 
corner is the main switch which opens and closes the motor cir- 
cuit. The switch in the upper left-hand corner controls the cir- 
cuits through the four magnets of the four remaining switches. 




fig. 215. 



Each one of these switches, when actuated by its magnet, cuts 
out a portion of the starting resistance in the armature circuit. 
As will be noticed, the starting resistance is cut out in four 
sections, while with switches of the types shown in previous 
chapters it is cut out in three or four times this number of sec- 
tions. It is generally considered that the greater the number of 



326 MOTOR STARTERS WITH ELECTROMAGNETIC SWITCHES. 

sections into which the resistance is divided, the smoother will 
be the acceleration of the speed of the motor in starting. This 






jy 




FIG. 2l6. 

conclusion is true theoretically, but in practice it is found that 
the difference in the smoothness of the acceleration when the 



MOTOR STARTERS WITH ELECTROMAGNETIC SWITCHES. 327 

number of sections into which the resistance is divided is four 
or five, or double or triple this number, is so slight as not to be 
noticeable. The fact is that no violent change in the speed of 
the motor can be effected unless nearly all the starting resist- 
ance is cut out at once, because the inertia of the armature and 
other moving parts resists any sudden change. 

In starters such as that shown in Fig. 211 the number of D 
contacts is made large, not for the purpose of securing smooth 
acceleration of velocity, but to reduce the sparking. 

In a starter such as is shown in Fig. 215, the only switch at 
which there is any tendency to spark heavily is the main starting 
switch, in the lower left-hand corner. When the motor is stopped, 
this switch opens the main circuit and, if the load on the 
motor at the time is large, the sparking may be severe. To 
reduce this sparking to the lowest point, magnetic blowouts are 
provided. The general operation of the starter can be well un- 
derstood from the following explanation of the diagram, Fig. 
216: 

When the main switch M is closed, the circuit from the N 
side of the main line passes through wire a to magnet A of the 
starting switch. From magnet A the circuit extends to binding 
post z and thence through wire p to the small switch y. By 
means of this switch y, which may be located at any desired 
point, the operation of the starter is controlled ; for, as can be 
seen, if 31 is open, the circuit is broken, while if it is closed, the 
circuit continues through wire p' to post 2, then through wire q 
to lever C of the B switch, to wire 0", to wire 0', and thence to 
the opposite side of the main line through wire b. 

This circuit being established by the closing of switch y, the 
magnet A of the starting switch will draw up its plunger and 
thus connect the contacts x x. Then the main circuit through 
the motor will be from wire b through the series field coils m m 
of the motor, through the motor armature to wire k', through 
the starting resistance H H to wire c and wire c, and thence 
through the connector of switch A to wire a and the opposite 
side of the main line. 

At the instant this circuit is closed by the closing of the start- 
ing switch A, a current will pass through wire u to magnet B, 
out through contact t to the small switch u', thence to the stud 



328 MOTOR STARTERS WITH ELECTROMAGNETIC SWITCHES. 

around which C swings, and through wires 0" and 0' to wire b 
and the main line P. Magnet B will now begin to draw up its 
plunger and thus swing lever C over the contacts E, the speed of 
the movement being regulated to any desired point by adjust- 
ment of the dashpot seen in Fig. 215. 

Circles L, V and L" represent incandescent lamps arranged 
so as to be cut into the circuits of the magnets A, B and G after 
these have raised their plungers to the top position. The circuits 
through the other three switch magnets are opened after they 
have performed their parts in the operation of starting the motor. 
When C is in the position shown, the circuit of magnet A from 
binding post z is through wire q to C and thence to 0" ' , but when 
C is moved to the second E contact, wire q is disconnected, and 
the circuit is then made through wires s' s' and lamp L,- thus 
cutting resistance into this circuit and reducing the strength of 
the current that passes through A. For every other position of 
€ above this, the circuit oi A is through wires // and lamp L. 

When C moves up as far as contact t" , the circuit through 
magnet D will be closed through wire t": The plunger of this 
switch will then be. raised, closing the circuit between wires h 
and c and thereby cutting out the top section of the starting 
resistance H H. When C advances to contact v, the circuit 
through magnet E will be closed through wire v' , and the plunger 
of this switch will be raised, connecting the wires i and d and 
cutting out the upper two sections of the starting resistance. 
When C passes onto v it opens the circuit through D and the 
plunger of this magnet drops, but this does not matter at this 
stage, as switch E now closes the circuit between wires i and d. 

When C reaches the contact s } the circuit of magnet F is 
closed through wire /', and the lifting of the plunger of ■- this mag- 
net closes the circuit between wires e and. //thus cutting out the 
upper three sections of the resistance H H. When C passes onto 
j, the circuits of magnets D and E are both opened. When C 
reaches contact r, the circuit through magnet G is closed through 
wire r" , and at the same time the circuit of magnet F is opened. 
Closing the circuit of G lifts the plunger and connects wires g 
and kj, thus cutting out the whole of the starting resistance H H. 

._ When this position is reached, the three magnets, D, E and 
F \ are cut out of the circuit, as they are no longer required, 



MOTOR STARTERS WITH ELECTROMAGNETIC SWITCHES. 329 

since magnet G makes the proper circuit connection. When C 
reaches the top contact, the lamp L" is cut into the circuit of 
magnet G through the wire r', and lamp V is cut into the circuit 
of magnet B, by a projection on C which actuates the small 
switch u , thus breaking the connection with contact t and forcing 
the current to pass through wires f t' and lamp L'. 

From the foregoing, it will be seen that the magnetic switches 
D, E and F are rendered active only during the short interval of 
time when they are used to cut out their respective sections of 
the starting resistance HH, and that immediately after they 
have performed their work the current through their magnets 
is cut off. When the motor is running at full speed, the magnets 
A, B and G are energized, but one of the lamps L is cut into 
each circuit, so that the current passing through the magnets is 
small, just strong enough to hold the plungers in the upper 
position. 

When the operating switch y is opened, the circuit through 
the magnet A of the starting switch is opened and the plunger 
drops, thus opening the main circuit through the motor. As 
soon as this circuit is opened the circuits through the magnets 
B and G are opened. As switch A opens the main motor cir- 
cuit, there may be considerable sparking between the connector 
and the contacts x x, if the motor is carrying a heavy load at the 
time, but this spark is broken by the action of the blowout 
magnets o o which are provided for that purpose. In any case, 
the sparking cannot be injurious, because the motor is so con- 
nected permanently in series with the armature that the circuit 
through the shunt field coils 5 ^ is never opened. That such is 
the case can be readily seen by tracing the motor circuit from 
the upper armature brush. This circuit runs through wire k' 
to resistance H H and into wire c", which connects with wire c, 
the latter, connecting with wire n to binding post z and from here 
through wire n to the left field coil 6" and out to wire n", thence 
through the series coils m m to wire b' and the lower motor 
armature brush and through the armature to the starting point. 
This is the connection with all the switches either opened or 
closed ; hence, unless the current passing through the motor is 
very strong when the machine is stopped, the sparking at the 
contacts of switch A will be small. 



330 TESTING ELECTRIC MOTORS. 



CHAPTER XLIV. 
Testing Electric Motors. 

TO BE able to make a test of a motor in any place and under 
any conditions, it is necessary to understand the principles 
upon which the test depends. These principles are simple 
and easily understood, and it is proposed to explain the subject 
fully in what fellows. 

In making a test of a motor, we can find out a number of 
things. We can ascertain the amount of electrical energy it ab- 
sorbs, and also the amount of work it delivers at the pulley. 
By deducting the last amount from the first, we can find what 
amount of energy is lost in transforming the electrical energy 
supplied to the motor into the mechanical energy it delivers at 
the pulley, and if we divide the latter energy by the electrical 
energy, we shall obtain the commercial efficiency of the machine. 
We can not only find out the proportion of the electric energy 
that is lost in the motor, but we can go further and ascertain 
how it is lost, what proportion is lost in the armature, what 
proportion in the field, and what proportion in other ways. We 
can, in addition to determining the amounts of energy absorbed 
and delivered, find the difference in efficiency of the motor for 
different percentages of load. 

Motors can be tested in two ways — by purely electrical meth- 
ods, or by a combination of mechanical and electrical methods. 
In this chapter we will explain the purely electrical methods. 

Electrical tests can be made in a simple manner and in a 
few minutes' time, but such are only accurate enough to give a 
fair idea of what the machine is doing. The simplest test of all 
for direct-current motors is. made with a single ammeter, to de- 
termine the efficiency of the motor and also the power it is de- 
veloping. For this test the ammeter is connected in the motor 
circuit, so as to measure the total current passing through the 
machine. The way in which the instrument is connected is fully 
explained in Chapter XXIII, Part I. 

Having connected the ammeter, the belt is thrown off arid 
the motor started up running light. When the starter has been 



TESTING ELECTRIC MOTORS. 331 

turned to the last point, and the armature is running at full 
speed, the ammeter is read, and the number of amperes indi- 
cated is noted. The motor is now stopped, the belt is put on, 
the machine started and full load applied. We now read the 
instrument for the second time. 

Suppose that the first reading is 10 amperes, and the second 
is 100 amperes, then we know that to run the machine light 10 
amperes are required, and that to drive the full load the current 
must be increased by 90 amperes. From this we might conclude 
that the electric current available to do work was 90 amperes, 
and that lost in the motor was 10, but this conclusion is not 
strictly correct. The truth is that, when the machine is driv- 
ing the full load, the loss in it is greater than when it is run- 
ning light. For the present, we can assume that when full 
load is on, it requires 12 amperes — that is, an increase of 2 per 
cent over the no-load loss — to overcome the loss within 
the machine, and the current available for doing work is 88 
amperes, so that the efficiency of conversion is 88 per cent; that 
is, we take into the motor electric energy equal to 100 and de- 
liver power, or mechanical energy, at the pulley equal to 88 per 
cent. 

This test, however, gives no idea of the amount of power 
the motor develops, because the number of amperes alone is no 
measure of electrical energy. To find the amount of electrical 
energy, we must know the potential or voltage of the current. 
If we have a voltmeter, we can connect it with the motor as 
explained in Chapter XXIV, Part I, and then by multiplying the 
volts indicated upon the instrument when connected with the 
motor terminals, by the current in amperes, we shall get the num- 
ber of watts of electrical energy given to the motor. 

Suppose that the voltage is 100, then the watts when the 
motor is running loaded and the current is 100 amperes will be 
100 X 100= 10,000. This is the total amount of electrical energy 
absorbed, but the portion of this energy that is transformed into 
useful mechanical energy and delivered at the pulley is 8,800 
watts, which is the product of 88 amperes by 100 volts. The 
energy lost in the motor is 1,200 watts. To find the amount of 
power delivered at the pulley, in horsepower, we divide the 8,800 
watts by 746, this number of watts being equal to 1 horsepower. 
If the division is made, it will give nearly 12 horsepower. 



33^ 



TESTING ELECTRIC MOTORS. 



If we are not provided with a voltmeter, we can come fairly 
near the real amount of energy by going by the voltage of the 
line; that is, the voltage it is supposed to have. Thus, if the 
motor is connected with the circuit from a lighting station that 
is operated at 220 volts, we can .ake this figure as approximately 
correct, and get an indication of the amount of power which 
will not be more than 6 per cent out of the way, because the 
actual voltage is not likely to be more than 230 nor less than 210. 

In order to make an accurate test, we first ascertain the re- 
sistance of the field coils of the motor and also that of the arma- 
ture. The resistance of these parts will not be the same when 



^>. i . 




FIG. 217. 

the wire is cold as when it is hot; the higher the temperature, 
the higher the resistance; hence, it is best to run the motor 2 or 
3 hours before measuring the resistance, so as to get the arma- 
ture and the field coils heated up to the temperature that they 
attain in actual running. 

To test the resistance of the coils, the terminals are discon- 
nected from each other, as illustrated in Fig. 217. The best way 
to test the resistance is by means of a galvanometer and bridge, 
as explained in Chapter XXVII. The Combination of a gal- 
vanometer and a bridge is commonly called a testing set, and 
by many it is known by no other name. 

If a testing set is not at hand, the resistance of the arma- 



TESTING ELECTRIC MOTORS. 



333 



ture and field can be ascertained by means of a voltmeter and an 
ammeter. This method will enable us to find the resistance of 
the field with a fair degree of accuracy, but for measuring the 
resistance of the armature it is practically useless, unless the 
voltmeter is calibrated to measure very small voltages, say from 
5 volts .down to a small fraction of I volt. 

To measure the resistance of either the armature or the. 
field coils by means of a voltmeter and an ammeter, the ter- 
minals are connected with the instruments and with a battery or 
other source of current, as shown in Figs. 218 and 219. In both 




fig. 218. 



of these diagrams B represents a battery, which, for testing the 
armature, should be a storage battery capable of giving a strong 
current, fully as much as is required to run the motor up to full 
power. The voltage should not be more than 3 per cent of that 
required to operate the motor. For testing the field coils, the 
battery need not give a current of more than an ampere, and 
in most cases considerably less, but the voltage should be about 
one-half that for which the motor is designed. Such a voltage 
cannot be obtained with batteries of any kind without connect- 
ing a large number of them in series, say from thirty to seventy. 



334 



TESTING ELECTRIC MOTORS. 



From the foregoing, it will be seen that it is not convenient 
to use batteries either for armature or field coil tests, and the 
best way is to use the same current that runs the motor, intro- 
ducing a sufficient amount of resistance to cut it down to the 
required strength. 

In Figs. 218 and 219 the connections are shown for testing 
the armature, but the connections for testing the field are pre- 
cisely the same. 

For the field the resistance can be found with a fair de- 




fig. 219. 



gree of accuracy by this method, because, as this resistance is 
high, the voltage with a small current will be high. To illus- 
trate, suppose that we connect the field as shown in these dia- 
grams, and find that the current is 2 amperes, and the volts 240; 
then, by dividing the volts by the amperes, we shall get the re- 
sistance in ohms, and 240 divided by 2 gives us 120, which is the 
number of ohms resistance in the field coil. 

As the resistance of the armature is very low — generally a 
few hundredths of an ohm — even with a strong current the volt- 
age is low. Thus, if the resistance of the armature is, say, 0.02 
ohm, and we pass through it a current of 100 amperes, the 



TESTING ELECTRIC MOTORS. 335 

volts will be only 2. If the voltmeter which we have is one 
that indicates 100 or more, we shall be unable to determine any- 
thing positive about the armature resistance with it, for it is not 
practicable to measure fractions of a volt with such an instru- 
ment. If, however, we have a voltmeter that will measure hun- 
dredths of a volt, and indicate as high as 5 volts, we can use it 
and determine the armature resistance fairly well, following 
the same rule as for the field coils — that is, divide the volts 
by the amperes of current, and the quotient will be the resist- 
ance in ohms. 

By these methods the resistance of the field coils can be 
measured to within less than 1 per cent, but that of the arma- 
ture cannot very well be ascertained much closer than 2 or 3 
per cent. By the use of the galvanometer and bridge, the re- 
sistance can be determined to within a very small fraction of 
1 per cent, say the one hundredth part of 1 per cent, so that it 
is by far the best method of testing, and generally there is no 
difficulty in obtaining a testing set. 

In measuring the armature resistance by means of a volt- 
meter and an ammeter, measurements should be made with the 
instruments connected as shown in both the diagrams, and 
then the average of these should be taken. As can be seen 
at once, if the voltmeter is connected as in Fig. 218, it will 
indicate the voltage absorbed by the ammeter as well as by 
the armature, and although the resistance of the ammeter is 
very low, its presence in the circuit will increase the voltage 
reading, on account of the strong current used. If the con- 
nections are made as in Fig. 218, the ammeter will indicate the 
current passing through the voltmeter as well as that passing 
through the armature, and if the instrument is intended for 
very low voltages, the current passing through it may be suffi- 
cient to slightly increase the reading of the ammeter. Gen- 
erally, however, this will not be the case, so that in most cases 
a single reading with the instruments connected as in Fig. 218 
will be sufficient. 

Having found the resistance of the field coils and that of 
the armature, we can determine the loss in these two parts 
separately, and the accuracy with which we determine these 
two losses will depend upon the accuracy with which we have 



27,6 TESTING ELECTRIC MOTORS. 

measured the resistance. Suppose that the resistance of the 
field coils is ioo ohms, and that current is supplied to motor 
at a voltage of 200, then by dividing this voltage by the field 
resistance we find that the current passing through the field 
coils is 2 amperes, and multiplying this current by the voltage, 
we get 400 watts as the energy absorbed in the field coils; and 
this loss will remain the same no matter whether the motor is 
running light, or fully loaded. 

Armature current will increase with the load, so that the 
loss due to the resistance of the armature coils will be small 
when the machine is running free, and will increase as the load 
increases. Suppose that the armature resistance is 0.02 ohm, 
and that, when the motor is running fully loaded, the armature 
current is 100 amperes, then the voltage absorbed by the arma- 
ture at full load will be 2 volts, being the product of the resist- 
ance by the current strength. Multiplying this voltage by the 
current, we get 200 watts as the loss due to armature resistance 
when running with full load. 

If we now make a test in the manner first explained — that is, 
by running the motor light and then fully loaded, and take the 
difference in the current strengths for the two cases — we shall 
find that it will be more than what is represented by the field 
coil and armature resistance losses combined. This difference 
represents the loss that is due to mechanical friction, and also 
magnetic friction. The mechanical friction is the bearing and 
armature brush friction and also the resistance of the air rub- 
bing against the rotating parts. The magnetic friction is that 
within the iron caused by the action of the particles upon 
each other as the metal is magnetized and demagnetized. This 
friction is called "hysteresis." 

It is difficult to separate the mechanical friction loss from 
the magnetic friction loss, or hysteresis loss, but generally they 
are about equal and are practically the same whether the motor 
is running light or loaded. Assuming this to be the case, we 
then have three losses that are constant, namely, the field coil 
loss, the mechanical friction loss, and the hysteresis loss. Now, 
if we run the motor light and measure the current, we know 
that of this current a certain amount passes through the field 
coils, and the balance goes through the armature, one-half be- 



TESTING ELECTRIC MOTORS. 



337 



ing absorbed by mechanical friction and' the other half by hys- 
teresis. The loss in the armature due to the passage of the 
current through it will increase in proportion to the square of the 
current. 

With these facts determined by our tests, we can draw a 
diagram such as is shown in Fig. 220, in which the figures .on 
the right-hand side indicate percentage of work utilized or lost, 
and those along the bottom indicate percentage of rated load. 





































e 




























33 










S> 






/ 


/ 














' 






















P 

/ 


/ 














/ 


/ 


'.." 


















/ 


/ 

74 
















e 


/ 
















/ 


/ 




















/ 
















/ 


/ 




E 
































/ 


/ 




























25 








/ 


' 


































P. 


/ 


/ 
19 






































































- 


9 








-a 


























^ 


^ 




A\ 




s 
















































































< 




















• 




1 














' 










































' 


















.^_ 


-V> 




^\ 


















rH 






















































































1 



O 10 20 30 40 50 60 70 80 90 tOO 

PE R CENT RATED LOAD 



FIG. 220. 



In this diagram the lower area, marked A, represents the energy 
lost in the field coils, which, as we have shown, is the same 
for all loads. The shaded area above this, marked B, represents 
the hysteresis loss, which is also practically constant; in fact, 
it is absolutely constant at the same speed. The unshaded area, 
C, represents the mechanical friction loss, which is also prac- 
tically constant. The shaded section above this, marked D, 
which begins at nothing on the left-hand side and becomes wider 
as it approaches the right-hand side, is the loss due to armature 
resistance, which is insignificant when the load is light, and in- 



338 TESTING ELECTRIC MOTORS. 

creases with increasing load. Thus we find that, if the motor is 
running without load, the total loss is about 7^ per cent, and 
when the full load is on it increases to 10 per cent. 

If the current absorbed by the motor running at full load is 
100 amperes, according to this diagram it will require 7.50 am- 
peres to run light, and at this point, as it does no work, all the 
energy it receives is lost; hence, the efficiency is zero, and the 
work done is zero. The curve e e represents the efficiency for 
all loads and, as will be seen, it is 25 per cent for 10 amperes, 
62 per cent for 20 amperes and continues to increase, being 88 
per cent for 75 amperes, and 90 per cent for 100 amperes. The 
line PP shows what portion of the total capacity of the motor 
is given with different strengths of current, this being zero for 
7.5 amperes, 19 per cent for 25 amperes, and 46 per cent for 50 
amperes. These are percentages of the full load capacity, which 
is 90 per cent of the electrical energy absorbed. To obtain the 
curves ee and PP, or the percentage figures marked upon them, 
all that is necessary is to add to the 7.5 per cent loss with no load 
the armature loss obtained by multiplying the square of the 
current by the armature resistance. The upper part of this 
diagram is drawn to a smaller scale, vertically, than the lower, 
so as not to make it too high. The No. 7 line is the zero line 
for the upper part of the diagram and, as will be noticed, 
curves ee and PP start from points on this line; to be strictly. 
accurate, they should start from curve a a. 



TESTING ELECTRIC MOTORS. 339 

CHAPTER XLV. 
Testing Electric Motors — (Continued.) 

IN THE last chapter we explained several methods of test- 
ing electric motors by means of electrical instruments. Such 
tests can also be made by using combinations of electrical 
instruments and mechanical devices. A common way of making 
tests by a combination of electrical and mechanical means is 
illustrated in Fig. 221. In this diagram, F represents the motor 
to be tested, B a fan blower that is driven by the motor, and D 
a dynamometer that is interposed between the fan and the motor 
to measure the power transmitted. 

Strictly speaking, the dynamometer D does not measure 
the power transmitted ; it simply indicates the force or pull on 
the belt, and to obtain the power it is necessary to multiply 
this by the velocity of the belt. Some dynamometers are cali- 
brated to indicate the pull upon the belt, so that to obtain 
the power in foot-pounds the velocity at which the belt travels 
in feet per minute must be multiplied by the reading of the 
instrument. Then, if this product is divided by 33,000, the horse- 
power is obtained. In other dynamometers, the calibration is 
such that the number of revolutions per minute is given instead 
of the belt speed. It is necessary, therefore, before making a 
test, to ascertain how the dynamometer is calibrated; the name 
plate on the apparatus usually gives the required informatfon. 

Starting from the top of the diagram (Fig. 221), the wires 
P N are connected with the supply circuit, and the ammeter A 
and voltmeter V enable us to measure the strength and voltage of 
the current, the product of these two readings giving the power, 
as has been explained in previous chapters. By adjusting the 
outlet gate of the fan B, the power required to drive it can be 
increased or decreased as desired, so that from the dynamometer 
D we can take measurements of the power delivered, and from 
the instruments A and V can determine the electrical energy 
absorbed for full load or for any portion of full load that we may 
desire. In this wav we can determine the relation between en- 



340 



TESTING ELECTRIC MOTORS. 



ergy received and energy delivered, or the efficiency of conversion, 
for several proportions of load. 




FIG. 221. 



In order that a test made in this way may be at all accurate, 
it is necessary that the; indications of the instruments A and V t 



TESTING ELECTRIC MOTORS. 34I 

and of the dynamometer D, as well as the velocity of D, or of 
the belt, as the case may be, be taken at the same instant, or as 
nearly so as possible; otherwise the results may be far from cor- 
rect. This liability of error from not taking the readings of the 
instruments at the same instant arises from the fact that the 
voltage of the current cannot be depended upon to remain abso- 
lutely constant, and any variation in it will cause a material dif- 
ference in the amount of energy supplied to the motor. 

As there is a possibility of making an error in reading. the 
indications of the ammeter and the voltmeter, and as both may 
not be read at the same time, it is advisable to use a wattmeter 
as a substitute for these two instruments if one can be obtained. 
This substitution of one instrument for two reduces materially 
the liability of making mistakes. A still better plan is to use an 
integrating wattmeter which will give a true record of the energy 
that passes through it during a given period. If such an instru- 
ment is used, and the test is made to cover a run of I hour, the 
record of the instrument will show the average power during the 
run. If, during this time, the indication of the dynamometer is 
taken every minute, we shall have 60 readings from which the 
average can be obtained by the simple process of adding them all 
together and dividing the sum by 60. To make the test as ac- 
curate as possible, the gate in the end of the blower should be 
undisturbed during the run, so as to maintain the power practi- 
cally constant. 

By a test made in the manner just explained we get the 
power delivered by the motor, and the energy absorbed, under 
actual running conditions, and from these two amounts we can 
obtain the running efficiency of the motor, as well as its actual 
capacity in horsepower. But we cannot separate the various 
losses in the motor, as may be done by means of the purely elec- 
trical tests explained in the last chapter. The^ blower B, as will 
be readily understood} may be replaced by any other kind of ma- 
chine, or by a number of machines. Thus if the motor is in 
actual service, the dynamometer D can be connected between the 
motor and the main shaft, the belt from the motor running to the 
dynamometer, and the belt from the latter to the line shaft. ' If a 
■test is made of a motor in actual service, it is desirable that dur- 
ing~tTre test the load be kept as nearly uniform as possible. 



342 



TESTING ELECTRIC MOTORS. 



In many cases, where it is not convenient to use a blower or 
any other kind of machinery to absorb the power of the motor, if 
we have a second motor, it may be used as a generator, and be 
driven by the motor to be tested. Then, by measuring the 
electric energy absorbed by the motor, and that developed by the 
generator, we can ascertain the capacity and also the efficiency of 
the machine. For this kind of test the two motors are arranged 
as shown in Fig. 222, motor M acting as a motor receiving cur- 



M 




FIG. 222. 



rent from the supply mains, while motor M' acts as a generator 
driven by M through the belt B. The current generated by M" 
can be utilized in a number of lamps as indicated, or it can be 
passed through another motor or through a resistance. The cur- 
rent supplied to M from the main circuit is measured in the 
same way as in Fig 221 ; that is, by the use of a wattmeter or an 
ammeter and a voltmeter. The current delivered by M r is meas- 
ured in the same manner, the instruments A and V representing 
an ammeter and a voltmeter which may be replaced by a watt- 
meter if such an instrument is available. 



TESTING ELECTRIC MOTORS. 343 

If the two motors M and M' are of the same size and make, 
we may assume that they are of equal efficiency, and by taking 
one-half the difference between the energy absorbed by M and 
that delivered by M', we shall come very near the loss in each 
machine. This method will not give us a perfectly correct re- 
sult because the current drawn from the supply circuit by M 
will be stronger than the current delivered by M', and the loss 
in M will, therefore, be greater than that in M' '; also the loss 
from slippage of belt is charged against the machines. 

If we desire to make the division of the loss more accurately, 
we can do so by ascribing to each machine a portion of the loss 
proportional to the energy absorbed or delivered by it. For ex- 
ample, suppose that M absorbs 10 kilowatts and that M' delivers 
8 kilowatts ; then if the difference between them, which is 2 
kilowatts, is divided into 18 parts, and 10 of these are given to M 
and 8 to M', we shall arrive at nearly the true result. Carrying 
this calculation further, we shall find that if M loses 10 parts, 
the total electric energy it develops is 10,000 watts less 10 times 

2,000 

-, or 1,110 watts, = 8,890 watts; from which we find that 

18 

the efficiency of M is about 89 per cent. 

If the two machines M and M' are not of the same make, or 
if they are of different sizes, we can approximate the efficiencies 
of both by making one test with M running as the motor and 
another test with M' as the motor and M as the generator. If 
there is a difference in the efficiency of the two machines, these 
two tests will not give the same results, so that by comparing 
them and striking an average, we can come very close to the 
actual efficiency of each machine. 

Another way of testing when we have two motors is to use 
the current generated by the second machine in driving the first 
one. For such tests, the motors are connected with each other 
as shown in Figs. 223 and 224, in both of which M and M' repre- 
sent the motors and B the connecting belt. This arrangement of 
motors for testing is extensively used in shops where they are 
manufactured, its advantages being that, with a comparatively 
small amount of power, large motors can be tested. 

In Figs. 223 and 224 the rectangle B represents a storage 



344 



TESTING ELECTRIC MOTORS. 



battery, or any other source of current, used to provide the extra 
power required to drive the motor. If M' is driven at the same 
speed as M, both machines being alike, the voltage developed by 
M' will be lower than that required to maintain M at the proper 
speed. If such is the case, a battery connected in series with the 
two machines, as in Fig. 223, will supply the additional voltage. 
With this arrangement, if we connect voltmeters and an 
ammeter as indicated in Fig. .223, we shall find that the voltmeter 




L£> QJn 



FIG 




D 



V will show a higher electromotive force than V", and that the 
difference between them will be the same as the indication of V\ 
thus showing that the battery B adds its voltage to that of M' so 
as to provide the requisite voltage to drive M at the proper speed. 
The voltage indicated by instrument V shows the loss in both 
machines, because the voltage delivered by M' is not the full 
voltage which it generates, but it is this voltage less the amount 
lost within the machine. In the same way, the voltage required 



TESTING ELECTRIC MOTORS. 



345 



to run M up to full speed is the amount required to impart to 
the armature this velocity plus enough to cover the loss within 
the machine. 

In Fig. 224 the battery B is connected in parallel with the 
generator M\ and in this case the voltage of both machines is 
made the same. This result is obtained by proportioning the 
sizes of the pulleys on M and M' so that the latter may run fast- 
er, the difference in speed being dependent upon the difference in 



&Jt& 





LS> Q^a 




FIG. 224. 



voltages at the same velocity. In this case, although M' will pro- 
vide the proper voltage, it will not furnish all the current re- 
quired, and to make up the deficiency the battery B is drawn 
upon. If the ammeters A and A' are examined, it will be found 
that the former indicates a considerably stronger current than 
the latter, the difference between the two being supplied by the 
battery. 

If the efficiency of the motors is 90 per cent, the loss of 



346 TESTING ELECTRIC MOTORS. 

energy in the two machines will be about 19 per cent, and from 
this it will be seen that the battery B will have to supply only 
about one-fifth of the current that would be required to drive M 
if it were supplied entirely from an external source. Thus with 
this arrangement, if we have a generator capable of developing 
10 horsepower, we may use it as a substitute for the battery B 
and be able to test motors of 50 horsepower capacity. Hence the 
general- use of this method in motor manufacturing shops. 



TESTING ELECTRIC GENERATORS. 347 

CHAPTER XLVI. 

TESTING ELECTRIC GENERATORS. 

TESTING an electric generator is fully as simple as testing 
an electric motor. In fact, the only real difference is that, 
in the case of the motor, we measure the amount of elec- 
trical energy supplied to the machine and the amount of mechani- 
cal energy it gives back, while in the generator test we measure 
the mechanical energy required to drive the machine and the 
electrical energy it gives back. In the first case, the difference 
between the electrical energy absorbed and the mechanical en- 
ergy given back shows the energy lost in the motor. In the sec- 
ond case the difference between the mechanical energy required 
to drive the machine and the electrical energy it gives back shows 
the portion lost in the generator. 

In the last chapter we outlined, in connection with the ex- 
planation of Fig. 222, the general method pursued in measuring 
the power developed by a generator; but to make the subject 
quite clear we present here in Fig. 225 a dia- 
gram showing the general arrangement for a complete 
test. In this diagram, S represents a line shaft from which 
the generator G is driven through the belts B and E. 
This shaft v? may be the shaft of a steam engine, or a line shaft 
driven from any source of power. The belt B runs over a pul- 
ley of a dynamometer D, from which the belt E transmits the mo- 
tion to the generator, the arrangement being precisely similar to 
that shown in Fig. 221. 

Dynamometer D enables us to measure the power required to 
drive the generator, and by means of the voltmeter V and the 
ammeter A, we find the electrical energy delivered to the circuit 
by the generator. The difference between the two amounts is 
the energy that is lost in the generator, and this, as in the case of 
a motor, is absorbed partly in the armature and partly in the 
field. The field loss consists of the energy absorbed in forcing 
the field current through the field coils, and is measured in pre- 
cisely the same way as the field loss in a motor — that is, by 



348 



TESTING ELECTRIC GENERATORS. 



multiplying the strength of the field current by the voltage meas- 
ured across the field terminals. 

Another way of finding the field loss is to measure the re- 




fig. 225. 



sistance of the field coils after they have become heated by a run 
of 2 or 3 hours, and then multiplying this resistance 
by the square of the field current, the product being the 



TESTING ELECTRIC GENERATORS. 349 

energy absorbed by the coils in watts. If we are not provided 
with an ammeter that will indicate a small current closely, we can 
calculate it by dividing the voltage of the generator by the re- 
sistance of the field coils, and then by multiplying the voltage by 
this calculated current we can get the watts lost in the field. 

In the armature of a generator the losses are the same as in 
the armature of a motor, and consist of the energy lost in the 
armature coils (which loss is determined in the same way as the 
field coil loss), the energy lost by mechanical friction, and that 
due to hysteresis, which is magnetic friction. These three arma- 
ture losses are substantially the same in magnitude as they are in 
the armature of a motor, so that, if we draw a diagram like Fig. 
220, the portions that represent the three armature losses and the 
one field loss will be the same as in the motor diagram. 

Resistances of the armature and field of the generator are 
obtained in the same way as in the motor, as explained in con- 
nection with Fig. 217. 

In testing generators, as well as motors, considerable work 
can be saved if we have wattmeters, as well as ammeters and 
voltmeters. Thus by connecting a wattmeter in the field coil 
circuit we can obtain the watts lost in the field by simply reading 
the dial of the instrument. If we connect a wattmeter in the main 
circuit, we can read on its dial the watts delivered to the external 
circuit, and thus save the trouble of multiplying the indication of 
the voltmeter V of Fig. 225 by the indication of the ammeter A. If 
the wattmeter is an accurate instrument, we shall be able to ob- 
tain more accurate results with it than with the ammeter and 
voltmeter, for the simple reason that in reading one instrument 
there is but one chance for making a mistake, while in reading 
two instruments there are two chances, and any mistake made in 
reading either instrument is magnified by being multiplied by the 
reading of the other instrument. 

In the last two chapters we explained only the course to pur- 
sue in testing shunt-wound motors, and what we have said up to 
this point in this chapter relates only to shunt-wound generators. 
At the present time nearly all stationary motors are of the shunt 
type, but generators are as a rule compound wound. The differ- 
ence between these two types, as has been explained in previous 



350 TESTING ELECTRIC GENERATORS. 

chapters, is that the field of the compound machine is magnetized 
by two sets of coils, one being the regular shunt coils, the other 
being a set of series coils through which all the current that flows 
through the armature is passed. 

In testing compound-wound motors, as well as compound- 
wound generators, all that is necessary in addition to what has 
been explained in connection with shunt machines, is to deter- 
mine the loss of energy in the series coils, and this is easily done 
by measuring the resistance of these coils, when heated by a run 
of several hours, and then multiplying this resistance by the 
square of the current that passes through the coils. 

If a compound-wound generator is well proportioned, it will 
be found that the loss of energy in the shunt coils of the field 
will be less than in a simple, shunt-wound generator, and that 
when the loss in the series coils is added to that in the shunt, the 
sum total will be about the same as in the simple shunt machine, 
or possibly a trifle less. 

Motors have series coils, in some cases, and are designated 
as compound-wound or as differential-wound motors, depending 
upon the way in which the series coils are connected. If the 
motor is compound-wound, the series coils are connected so that 
the current flowing through them runs in the same direction as 
that in the shunt coils, and in that case the series coils help the 
shunt coils to magnetize the machine. If the motor is differ- 
ential-wound, the series coils are so connected that the current 
flows through them in the opposite direction to the current flow- 
ing through the shunt coils, and in that case the series coils act 
in opposition to the shunt coils ; that is, they demagnetize the 
machine, so that the net strength of the field magnets is due to 
the difference between the magnetizing effects of the series and the 
shunt coils. It is on this account that this method of connection 
is called a differential winding. In a compound-wound motor the 
effect of the series coils is to cause the speed to drop faster than 
with the simple shunt coils, when the load is increased. The 
effect of the differential winding is to cause the speed to drop less 
when the load is increased. The compound winding is used on 
motors for the purpose of giving a strong turning effort or torque 
with a comparatively small current. This winding is commonly 
used for elevator motors and for other purposes where the ma- 



TEbTING ELECTRIC GENERATORS. 351 

chine has to start up under full load. In such cases, a simple 
shunt motor will take an excessively strong current to start, 
because the field is comparatively weak ; but if a compound 
winding is used, the field will be very strong because the full 
armature current will pass through the series field coils and 
thus greatly reinforce the action of the shunt coils. A shunt 
motor can be made so as to start up under full load with a 
current no greater than compound motors generally require, 
but if so made means must be provided to cut down the field 
current after starting in order to enable the motor to run up to 
full speed. 

In a differential-wound motor, the starting current under a 
full load is much greater than in a simple shunt machine, because 
the series coils act to demagnetize the field, and on that account 
the torque, or rotative force of the armature, for a given 
strength of current is considerably reduced. The differential 
winding, however, will cause the motor to run at a more uni- 
form speed, because when the load increases the field becomes 
weaker, and on that account the armature has to make more 
turns in a given time to develop the counterelectromotive force 
required to balance the line voltage. 

This advantage of the differential winding in the way of 
producing more constant speed is not as great as might appear, 
since the series coils, while acting to reduce the voltage devel- 
oped by the armature for each revolution, also act to reduce the 
total counter voltage required, on account of their absorbing a 
considerable portion of the line voltage. Because of the fact 
that designers are able nowadays to obtain about as close regu- 
lation of speed with the simple shunt winding as can be obtained 
with the differential, the latter type is seldom used in modern 
machines. 

Between motors and generators there are some relations that 
can be easily explained by the aid of four simple diagrams, Figs. 
226 to 229. 

One of the most important things to fully understand is that 
there is no difference whatever in the principle of action or con- 
struction between a motor and a generator. In the actual ma- 
chines there are, generally, some slight differences in the details 
of design, but these are made simply that each type of machine 
may be better adapted for the class of work it has to perform. 



352 



TESTING ELECTRIC GENERATORS. 



If allowed to run free, a simple shunt-wound motor will attain a 
speed sufficient to cut the armature current down to a value so 
low that the energy passing through the armature is just enough 
to overcome the electrical and magnetic losses and the mechanical 
resistance to rotation. If, at this point, we put a belt on the pul- 
ley and apply power by means of it so as to increase the speed of 
the motor, the result will be that the current passing through the 
armature will be still further reduced, and if we continue to in- 
crease the speed, the current will keep on reducing until it be- 
comes zero. 

Current reduces as the speed is increased because the arma- 
ture of the motor develops a voltage in the opposite direction to 
that of the line current, and this acts as a back pressure to hold 



p 

t 




Jv 




M 


/< 




m 






*— 1 


c& 


\^ 



FIG 226. 




FIG. 227. 



the line current back. This voltage, which is called the counter- 
electromotive force of the motor armature, or sometimes the 
back pressure, puts forth an effort to set a current flowing 
through the armature circuit in the opposite direction. The back 
pressure increases as the armature speed increases, and at a 
velocity slightly above that at which the motor will run free the 
back pressure becomes equal to the line voltage, so that the cur- 
rent flowing through the armature is reduced to nothing, because 
the two forces just balance each other. 

If now the armature speed is further increased, the back 
pressure will become greater than the forward, or line, pressure, 
and as a result the armature of the motor will generate a current 
that will flow back into the main line. Thus it will be seen that if 
we increase the velocity of the motor sufficiently, we convert it 
into a generator. 



TESTING ELECTRIC GENERATORS. 



353 



From the foregoing it will be seen that a shunt-wound motor 
without any changes in the wire connections, or in the direction 
of rotation, becomes a generator, if we only increase the velocity 
at which the armature rotates. By looking at Figs. 226 and 227 
it can be seen why no changes in the connections or direction of 
rotation are required. 

In Fig. 226 it must be remembered that the machine is acting 
as a motor, and that the line current comes in through the posi- 
tive wire P, hence it will branch through the shunt field coil m 
and through the armature A in the direction indicated by the 



P 

M 



t^y 



s 

FIG. 228. 




arrow heads. In Fig. 227, when the machine is acting as a gen- 
erator, the current comes from the armature A and it flows 
through this in the opposite direction, being driven by the back 
pressure. Now this current, when it reaches the shunt field coil 
m, will flow through it in the same direction as the line current 
did, when the machine was running as a motor, as is clearly 
shown by the arrow heads; but passing out into the main cur- 
rent wires P and N, it will flow against the line voltage; that is, 
the motor now feeds current into the main line instead of draw- 
ing from it. If the direction of rotation of the armature is re- 
versed, when the motor is acting as a generator, no current will 



354 TESTING ELECTRIC GENERATORS. 

be generated unless the connections of the field coil are also re- 
versed. 

If a differential-wound motor is driven above speed by the 
application of power, it will become a compound-wound genera- 
tor, as can be seen by comparing Figs. 228 and 229, the first show- 
ing the series coil S connected so that the current flows through 
it in the opposite direction to that through the shunt coil m; that 
is, in the direction of a differential winding. In Fig. 229, which 
shows the direction of field currents through both field coils, 
when the machine is running as a generator, it will be seen that 
in both coils the direction of current is the same; hence, a 
differential-wound motor, when driven above speed, becomes a 
compound-wound generator, and conversely a compound-wound 
motor, when driven above speed, becomes a differential-wound 
generator. As in the case of the shunt-wound motor, 110 change 
is made either in the direction of rotation or the wire connections 
to convert the motor into a generator, a slight increase in speed 
being all that is required. 



STORAGE BATTERIES. 355 



CHAPTER XLVIL 

STORAGE BATTERIES. 

STORAGE batteries are simply devices which transform elec- 
trical energy into chemical energy and vice-versa. They do 
not store electrical energy, because such a thing is impossi- 
ble. Electricity is simply a force of nature; it is not a material 
thing that can be bottled up. To charge a storage battery an 
electric current is passed through it; this current produces a 
chemical action which leaves the contents of the battery in what 
may be called an unnatural chemical state, and, as a consequence, 
they will restore themselves to the natural state as soon as the 
conditions are such that they can, and in this restoration an elec- 
tric current will be generated. 

The amount of electrical energy put into a storage battery is 
more than that which can be recovered from it, because a por- 
tion of the energy is absorbed in overcoming the resistance that 
opposes the passage of the current. This resistance hinders the 
flow of current when the battery is being charged, and also when 
it is being discharged, so that there is a loss in both operations. 
If the battery is allowed to stand but a short time after being 
charged, and is charged and discharged at a moderate rate, the 
loss will not be more than 10 per cent; but if the charging and 
discharging are both forced — that is, if the battery is charged and 
discharged in a short time — the loss may be much greater, possi- 
bly as much as 50 per cent. 

When a storage battery is fully discharged (in a practical 
sense), its energy is not entirely exhausted; it is simply run down 
to a point beyond which it is not advisable to carry it in prac- 
tice. A storage battery might be compared to a water pail hav- 
ing a sponge fastened to its bottom. If the pail is filled with 
water and then emptied, it will not give out all that was put into 
it, because the sponge will soak up some of the water. If it re- 
quired 10 quarts to fill it, and the sponge retained 2 quarts, then 
on pouring out the water only. 8 quarts would be obtained. A 
greater amount of water could be forced out of it by squeezing 
the sponge. If the pail is filled the second time, it will require 



356 STORAGE BATTERIES. 

only 8 quarts because the sponge, being full, will not soak up 
any more; so that when emptied the second time, as much water 
will be poured out as was poured in. 

From this it will be seen that after the first filling, all that 
will be lost will be the power required to fill the pail with water 
and to empty it. This is precisely the case with the storage bat- 
tery after it is once charged ; all that is lost in the successive 
charging and discharging is the power absorbed by the electrical 
resistance. 

Storage battery cells have an e. m. f. of about 2 volts each. When 
fully charged, the voltage is about 2.1, and when discharged it is 
about 1.8. In practice it is found that storage battery cells do not 
work well when connected in parallel, owing to the fact that 
when so connected some of the cells will give a stronger current 
than the others, and thus run down sooner. On that account, the 
cells are made of such size that one will have all the current 
capacity required. Thus, if the maximum demand of the circuit 
is 10 amperes, the cells will be made of such size as to deliver 10 
amperes, and if the demand is for i ; ooo amperes, each cell will 
be capable of delivering that number of amperes. The voltage re- 
quired is obtained by connecting a sufficient number of cells in 
series; for example, if the voltage required is 100, about fifty 
cells will be used. 

Small storage batteries can be placed on shelves secured to 
the wall, but with batteries of the sizes ordinarily used in con- 
nection with electric lighting plants, they must be placed upon the 
floor or in strongly constructed racks, as they are too large and 
heavy to be safely held on shelving. Where floor space is not 
cramped, the best arrangement is to locate the cells in a single 
tier, but if there is a scarcity of room they can be placed two or 
even three tiers high, being supported by strong framing made 
either of iron or wood. If the framing is of iron, strong insula- 
tors must be provided to hold the cells so that they may be well 
insulated from the ground. 

When the cells are placed directly upon the floor, wooden 
stringers are provided, as is illustrated in Figs. 230 and 231 at A. 
The first figure is a side view of a number of cells and the sec- 
ond is an end view of two rows. The wooden supports should be 
beams about 4 by 8 inches, set on edge, and should be well im- 



STORAGE BATTERIES. 



357 



pregnated with oil or paraffin to make them water proof. In 
addition to this treatment, they must be covered with a good coat- 
ing of coal tar, so that they may not be affected by the acid that 
is likely to be dropped upon them from time to time. The plates 



a 



a 




in the cells are provided with lugs by means of which they are 
connected with the plates of adjoining cells; and the distance 
between the cells must be such that these lugs may be properly 
connected, as is shown at a a in Fig. 230. 

If the length of the room is such that all the cells required 




f:g. 231. 



can be placed in two rows, they can be arranged with a passage- 
way between them; that is, one row on each side of the room> or 
they can be placed in the center of the room with a passage on 
each side. The first arrangement is the more desirable, if the 
room is narrow. If the room is wide and short, so that the cells 



358 STORAGE BATTERIES. 

have to be placed in more than two rows, then they should be 
set in pairs of rows, with a passageway between each pair. 

Storage batteries are used to increase the voltage when for 
any reason it is required to feed a current of higher e. m. f. than 
the normal into some branch of the circuit. For example, sup- 
pose that in a lighting plant where the normal voltage is no it is 
desired to have a current of 150 volts for some particular pur- 
pose; then a storage battery capable of furnishing the extra 4c 
volts is provided and the generator current is passed through the 
battery so that the voltage of the latter may be added to it. And 
in this way the no volts of the generator, plus the 40 volts of the 
battery, will give the 150 volts required. If the high-voltage cur- 
rent is not required all the time, the battery is charged by the 
generator while the high-voltage circuit is shut down. If the 
high-voltage current is required during all the time the plant is 
running, or for nearly all the time, two sets of batteries will be 
used, and one will be charged while the other one is being use'd. 

The most common and profitable use to which storage bat- 
teries are put is as a help to the generators. To illustrate their 
advantage in such cases, suppose that we have a lighting plant 
used in a factory to furnish light for an hour or less in the morn- 
ing, and a similar length of time in the evening. Let the maxi- 
mum number of lights used be one thousand ; then it is evident 
that a generator of one thousand-light capacity must be installed, 
and power sufficient to drive it must be provided. If the lights 
are used for 1 hour in the morning and 1 hour in the evening, 
the generator will be in service for only 2 hours out of the 10. 
If a storage battery is provided, and this is charged during the 
remaining 8 hours, it will have to be charged at a rate only slight- 
ly more than two hundred and fifty lights ; for to feed this num- 
ber of lights for 8 hours will require just the same amount of 
energy as to feed four times the number for 2 hours. This being 
the case, with the help of the storage battery a generator of 
three hundred-light capacity will be sufficient to do the work, and 
the steam engine capacity will be reduced in like proportion. 

In nearly all the large electric lighting stations, storage bat- 
teries are used. Old stations that were not provided originally 
with batteries install them when the demand for lights becomes 
so great that they cannot meet it with the generators running up 



STORAGE BATTERIES. 359 

to full capacity. In all lighting stations the demand for lights is 
not uniform throughout the 24 hours. It is heavy from 5 to 11 
o'clock in the evening and from about 6 to 8 o'clock in the morn- 
ing. During the day hours it is light, and from midnight to 6 in 
the morning still lighter. 

During the hours of light demand, the storage battery is 
charged, and when the heavy load comes on, the battery is con- 
nected so as to discharge into the circuit and help the generators. 
In this way the capacity of the station is considerably increased, 
for to the maximum capacity of the generators is added the ca- 
pacity of the battery. Another advantage of the battery is that, 
if for any reason the generators have to be shut down for a half 
hour or so, the battery can furnish the current, and thus avoid 
extinguishing the lights. 

Diagrams 232 and 233 show the way in which batteries are 
connected so as to be used to assist the generators in supplying a 
system of lighting for either a private or public plant. In both 
these arrangements the battery can be charged while the lamp cir- 
cuits are being fed, and when it is charged it can be connected to 
the lamp circuits and work together with the generator or alone, 
as the case may require. 

As stated in the foregoing, the voltage of battery cells varies 
from about 2.1 down to 1.8 volts, between full charge and dis- 
charge. Owing to this change in the voltage, the number of cells 
connected in series will have to be more when the battery is 
nearly discharged than when it is fully charged, so as to keep the 
line voltage up to the proper point. In charging a battery the 
voltage of the charging current has to be increased as the charg- 
ing progresses, so as to force a current through the cells against 
their constantly increasing voltage. On this account the number 
of cells connected in series has to be reduced as the charging in- 
creases, otherwise the generator electromotive force would not 
be able to set up current to charge the battery. 

To obtain the necessary adjustment the battery is divided 
into two parts, one called the main battery, which is shown 
at B in the diagrams, and the other, the end regulating cells, 
shown at B'. 

In both the diagrams the generator is represented by A and 
M, the former being the armature, and the latter the field coils, 



360 



STORAGE BATTERIES. 



and R is the field regulator. At A' an ammeter is placed to indi- 
cate the strength of the generator current, and at A" is another 
ammeter that indicates the strength of the current flowing 
through the battery. The double throw switch g is for the pur- 




fig. 232. 

pose of reversing the current through A" when the battery is 
being charged, but if an instrument is used which indicates with 
current flowing in either direction, this switch is not required. 
In Fig. 232, V is a voltmeter which, by means of the switch D, 



STORAGE BATTERIES. 



36l 



can be connected with the generator, the battery or the 
bus bars L L ', and thus show the voltage of any of these. In 
Fig. 233, three voltmeters are provided, which is a more con- 




fig. 233. 

venient but more expensive arrangement. The safety fuses or 
circuit breakers are shown at f and /'. 

In Fig. 232, if the switch r is open, as shown, the circuit will 
be fed from the batter}- ; by moving the switch S, more or less of 
the regulating cells in B' can be placed in the circuit so as to 



362 STORAGE BATTERIES. 

obtain the proper voltage. When the switch r is closed, the gen- 
erator will send a current into the circuit, and if switch S is 
now turned far enough to the left the generator current will be 
forced through the battery and will charge it, provided switch n 
is closed. The current passing through the battery from the 
generator will reach bus V and from there return to the genera- 
tor. By moving switch S far enough to the right, the number of 
end regulating cells in B' added to the battery can be made suf- 
ficient to cause the battery voltage to equal that of the generator, 
and then the battery current will join that from -the generator 
and flow out to the lamps. Thus it will be seen that by chang- 
ing the position of switch 5 the battery can be either charged or 
discharged while the generator is feeding the lamps. 

The difference between Figs. 232 and 233 is in the way in 
which the end-regulating cells are connected in the circuit. In 
the latter figure, with switch h in the position shown and switch 
n closed, the generator current will have to pass through all the 
end cells B' to reach the lamp circuits ; while by passing through 
the main battery B it can return to the starting point. From this 
it will be seen that by moving the switch S so as to increase or 
decrease the number of cells in B' the proportion of current pass- 
ing through the battery and out to the lamps can. be varied. 
When switch h is turned so as to connect with j, the generator 
current will pass out directly to the line the same as when switch 
r in Fig. 232 is closed. In either arrangement, the current flow- 
ing through the battery can be adjusted by the movement of 
switch S. 



INDEX. 



INDEX. 
A. 

Page. 

Action of short circuited motor armatures ' 256 

Ammeter and voltmeter for testing resistance 332 

use for testing motor efficiency 330 

Armature, action of short circuited motor 256 

connection, repairing broken 267 

connection, sparking caused by broken 264 

■ dynamo action with broken connection 266 

Armatures, finding and repairing short circuits 255,256 

Armature loss 335 

■ motor action with broken connection in 264 

out of center, effects on distribution of magnetism, 

bearing friction, and e. m. f 213,215 

resistance, measurement of 332 

resistance method of speed control 279, 305 

short circuits, repairing 261 

Armatures, finding and repairing broken connections 263 

grounds in 246 

wave winding for series connected multipolar 244 

B. 
Batteries, setting and mounting of storage 356 

storage 355 

Battery cells, e. m. f. of storage 356 

cells, end regulating 359, 362 

connecting storage to electric system 360 

use for increasing voltage and for heavy load hours. . 358 

Bearing friction, effect of armature out of center on 213 

Blower and dynamo for load in motor testing 341 

Blow-outs to stop sparking on controller switches. .... .320, 327 

Brake, electric for stopping motors 307 

Breaks in commutator connections 263 

Bridge, Wheatstone 207 

Broken connection, action of motor armature 264 

Burning of commutator due to broken connection 266 

C. 

Care of electrical machines 213 

Changing speed of motors ". 2^] 

voltage of generators 239 

Characteristics of generators, form for machines to be run 

in parallel, how to obtain 228, 231 

Commutator, burning due to broken connection 266 

connection, breaks in 263 

Compound or series coil, action, reason for use and strength 

of coil advisable 224 



11 INDEX. 

Compound-wound dynamos, connection in parallel 235 

-wound generators, equalizing connection for 236 

— wound motors 313, 350 

Connection, dynamo action with broken armature 266 

■ of compound wound dynamos in parallel 235 

of lamps as ground detector 247 

of motors and dynamos, relation of 352 

of multipolar dynamos for two-circuit armature 244 

— of shunt wound dynamos in parallel 232 

of shunt wound motors to line wires 269 

of starting boxes for motors 

269, 273, 287, 291, 293, 296, 300, 326 

repairing broken armature 263, 267 

— sparking caused by broken 264 

to determine whether dynamos can be run in parallel. 226 

Connections, break in commutator 263 

for motor controllers ....286, 304, 306, 310, 313, 317, 321 

of rheostats 222 

of speed controllers for motors 278, 283 

of storage battery into system 360 

Constant current and constant potential generators 219 

Control of motor speed by field and armature resistance. 279, 305 
of motors by magnetic switches 321 

of motors by push button 319 

Controller switches, blowout coils to stop sparking on... 320, 327 
Controllers and starters for motors .279, 284, 285 

for motors 278, 283 

for motors, connections of. 286, 304, 306, 310, 313, 317, 321 

for printing press motors 316 

— reversing for motors 309 

Counterelectromotive force 282, 352 

Current, constant, dynamos for „ 219 

deflection of magnetic needle by 205 

detection by galvanometer 206 

D. 

Differential-wound motors .350, 354 

Distortion of field cause of sparking. 215 

Distribution of magnetism affected by armature out of center 213 

Dynamo, action with broken connection in armature 266 

Dynamometer, measurement of power 339, 347 

Dynamos and motors, relation of connections 352 

care of 213 

characteristics of 228, 231 

connection of compound wound in parallel 235 

connection of shunt wound in parallel 232 

constant current and constant potential 219 

equalizing connection for 236 

plotting characteristic curves .231 



INDEX. Ill 

Dynamos, to determine whether suitable for parallel connec- 
tion 226 

testing electric 347 

two-circuit connection for multipolar 244 

E. 

Efficiency, ammeter for testing motor 330 

and loss curves 237 

of motor, interconnected method for testing 343 

Electric brake for stopping motors 307 

motors, testing of 330 

Electromotive force, effect of armature out of center on.... 215 

E. m. f . of storage battery cell 356 

Electromotive force, use of rheostats for regulating. .. .220, 239 

End regulating battery cells 359, 362 

Energy used in field coils of motor 270 

Equalizer connection for compound-wound generators run in 

parallel 236 

F. 

Field and armature resistance control of motor speed. . . .279, 305 

circuit, cause of sparking at switch 27s, 275, 291 

coils of motor, energy used in 270 

coils, repairing short circuits 253 

coils, short circuits in 246, 251 

distortion effect on sparking 215 

resistance, measurement of 332 

short circuit cause of sparking 251 

strength and voltage, relation of 241 

relation to motor speed 280, 282 

winding loss 335 

Friction loss in motors 336 

Fuses and magnetic cutouts for motor starters 271, 298 

G. 

Galvanometer, detection of current by 206 

measurement of resistance by 206 

principle and use 205 

use for finding short circuits -251, 259 

use with Wheatstone bridge for measuring resistance. 207 

Generators, connecting in parallel 232 

in parallel, method of shutting down 238 

in parallel, pump analogy 226 

methods of changing voltage - 239 

Ground detector of lamps, connection on switchboard and use 247 

on motor circuits 249 

Grounds, and short circuits in field coils 246 

H. 

Horsepower, relation to power in watts 33T 

Hysteresis loss 336 



I. 

Interconnected method for testing motor efficiency with 

dynamo load 343 

L. 

Lamps used as ground detector. 247 

Loading motor for testing 341 

Loss and efficiency curves 337 

Loss by hysteresis 336 

in armature and field windings of motor 335 

in motors from friction 336 

M. 
Magnetic cutout, use in motor starters .271, 298 

motor controllers 321 

needle, deflection by electric current 205 

— switch motor starters 324 



Magnetism, effect of armature out of center on distribution. . 213 

Measurement of power by dynamometers 339, 347 

of resistance by galvanometer 206 

of resistance by Wheatstone bridge and galvanometer 207 

Mechanical friction loss in motor 336 

Motor action with broken connection in armature 264 

armature, action when short circuited 256 

circuit, test for grounds 249 

controlled by push button 319 

controllers, magnetic 321 

energy used in field coils 270 

friction loss 336 

loss in armature and field winding 335 

speed and field strength 280, 282 

speed and voltage 281 

speed control by field and armature resistance. . . .279, 305 

starters and speed controllers 279, 284, 285 

starters, fuses and magnetic cutout for 271, 298 

starters, no voltage ......287, 288, 293, 305, 307, 312, 318 

starters, overload 293, 295, 320 

starters, sparking on ... 327, 329 

starters with magnetic switch 324 

starting boxes, connection for 

269, 273, 287, 291, 293, 296, 300, 326 

and dynamos, relation of connections 352 

changing speed of 277 

compound wound 313, 35o 

connection of controllers for 

286, 304, 306, 310, 313, 317, 321 

connection of shunt-wound to line wires 269 

connections of speed controllers 278, 283 

controllers for printing , presses 316 

differential wound 350, 354 



INDEX. V 

Motors, electric brake for stopping 307 

reversing controllers for 309 

reversing switch for .'. . . 274 

series wound, change of speed with load 277 

shunt wound, change of speed with load 281 

testing electric 330 

Mounting storage batteries 356 

Multipolar armatures, wave winding for series connected.. 244 
Multipolar dynamos, two-circuit connection 244 

N. 

Needle, deflection of magnetic by electric current 205 

No-voltage motor starters 287, 288, 293, 305, 307, 312, 318 

O. 

Overcompounding 225 

Overload motor starters 293, 295, 320 

P. 
Parallel connection of compound wound dynamos 235 

connection of generators 232 

connection of shunt wound dynamos 232 

connection, to determine whether generators are suit- 

able 226 

running of dynamos determined by characteristics 228, 231 

running of generators, shutting down 238 

Plotting characteristic curves of dynamos 231 

Potential, constant, dynamos for 219 

Power measurement by dynamometers 339, 347 

Principle and use of the Wheatstone bridge 207 

Principles and use of galvanometer 205 

Printing press motor controller 316 

Push button motor control 319 

R. 

Ratio arms of Wheatstone bridge 209 

Regulating end cells for battery 359, 362 

Relation of horsepower and watts 331 

of motor and dynamo connections 352 

of voltage to field strength 241 

of voltage to motor speed 281 

of voltage to speed 241 

Repairing armature short circuits 255, 256, 261 

broken armature connections 263, 267 

field coil short circuits 253 

Resistance in armature and field circuits to control motor 

speed r 279, 305 

measurement by galvanometer 206 

measurement by Wheatstone bridge and galvanometer 207 

Resistances of armature and field, measurement by ammeter 

and voltmeter and by Wheatstone bridge 332 



Reversing controllers for motors. .-. 309 

switch for motors 274 

Rheostats, construction and connections 222 

Rheostat, use for regulating e. m. f 220, 239 

S. 

Series coil, action of 224 

connected multipolar armatures 244 

wound motors, change of speed with load 277 

Setting and mounting of storage batteries 356 

Short circuit in field cause of sparking 251 

circuited motor armature, action of 256 

circuits in armatures 255, 256 

circuits in field coils 246, 251 

circuits, repairing in armature 261 

circuits, repairing in field coil 253 

circuits, use of galvanometer for finding : . .251, 259 

circuits, uge of voltmeter for finding 251, 258 

Shunt-wound dynamos, connection in parallel 232 

-wound motor, connection to line wires. . . 269 

Shutting down generators run in parallel 238 

Sparking due to broken wire in the armature 264 

due to field distortion : 215 

due to field short circuit 251 

of switch contact when opening field circuit. 273, 275, 291 

on motor starters 327, 329 

Speed and load in shunt wound motors 281 

and load, relations of for series wound motors 277 

and voltage, relation of 241 

control of motors by field and armature resis- 
tance 279, 305 

controllers for motors 279, 284, 285 

controllers for motors, connections of 278, 283 

of motors and field strength, relation of ....... .280, 282 

of motors and voltage, relation of 281 

— — — of motors, changing 277 

Starters for motors 279, 284, 285 

for motors, use of fuses and magnetic cutout. .. .271, 298 

— no voltage for motors 287, 288, 293, 305, 307, 312, 318 

— overload for motor 293, 295, 320 

Starting boxes for motors, connection of .• 

269, 273, 287, 291, 293, 296, 300, 326 

Storage batteries 355 

batteries, setting and mounting of . 356 

battery cells, e. m. f. of 35^ 

battery, connection to electric system 360 

Strength of field and motor speed, . . 280, 282 

Switch contact for field circuit, sparking of 273, 275, 291 

reversing for motors 274 



T. 

Test for grounds on motor circuit, with voltmeter 249 

for short circuit in field coils 251 

Testing armature and field resistance 332 

electric generators 347 

electric motors 330 

motor efficiency by the use of an ammeter 330 

motors by interconnected method 343 

motors, methods of loading 341 

sets 210 

Two-current connection for multipolar dynamos 244 

V. 

Voltage, battery used for increasing 358 

changing of generator 239 

relation to field strength 241 

relation to motor speed 281 

relation to speed 241 

Voltmeter and ammeter for testing resistance 332 

use for finding short circuits 251, 258 

use of to detect grounds 249 

W. 

Watts power, relation to horsepower 331 

Wave winding for series-connected multipolar armatures.... 244 
Wheatstone bridge, principle and use with galvanometer for 

measuring resistance . 207 

bridge, ratio arms 209 

Winding for series connected multipolar armatures 244 

loss for armature and field of motor 335 

of compound or series wound dynamo 224 



For Index to Part I. see end of Part I. 



NOV II I 



